mtp: Rename link_data to mtp_link and move out the transport specific things

Rename link_data to mtp_link and move it into the mtp_data header
file, also remove the union to ease creating more of the subtypes.

This is done in preparation to the linkset knowing more about the
link (e.g. having a link test per link instead of per link).
diff --git a/src/link_udp.c b/src/link_udp.c
index e05bd17..09b680f 100644
--- a/src/link_udp.c
+++ b/src/link_udp.c
@@ -37,18 +37,18 @@
 
 static int udp_write_cb(struct bsc_fd *fd, struct msgb *msg)
 {
-	struct link_data *link;
+	struct mtp_udp_link *link;
 	int rc;
 
-	link = (struct link_data *) fd->data;
+	link = fd->data;
 
 	LOGP(DINP, LOGL_DEBUG, "Sending MSU: %s\n", hexdump(msg->data, msg->len));
-	if (link->pcap_fd >= 0)
-		mtp_pcap_write_msu(link->pcap_fd, msg->l2h, msgb_l2len(msg));
+	if (link->base.pcap_fd >= 0)
+		mtp_pcap_write_msu(link->base.pcap_fd, msg->l2h, msgb_l2len(msg));
 
 	/* the assumption is we have connected the socket to the remote */
 	rc = sendto(fd->fd, msg->data, msg->len, 0,
-		     (struct sockaddr *) &link->udp.remote, sizeof(link->udp.remote));
+		     (struct sockaddr *) &link->remote, sizeof(link->remote));
 	if (rc != msg->len) {
 		LOGP(DINP, LOGL_ERROR, "Failed to write msg to socket: %d\n", rc);
 		return -1;
@@ -59,7 +59,7 @@
 
 static int udp_read_cb(struct bsc_fd *fd)
 {
-	struct link_data *link;
+	struct mtp_link *link;
 	struct udp_data_hdr *hdr;
 	struct msgb *msg;
 	int rc;
@@ -72,7 +72,7 @@
 	}
 	    
 
-	link = (struct link_data *) fd->data;
+	link = (struct mtp_link *) fd->data;
 	rc = read(fd->fd, msg->data, 2096);
 	if (rc < sizeof(*hdr)) {
 		LOGP(DINP, LOGL_ERROR, "Failed to read at least size of the header: %d\n", rc);
@@ -120,7 +120,7 @@
 	return rc;
 }
 
-static int udp_link_dummy(struct link_data *link)
+static int udp_link_dummy(struct mtp_link *link)
 {
 	/* nothing todo */
 	return 0;
@@ -128,38 +128,46 @@
 
 static void do_start(void *_data)
 {
-	struct link_data *link = (struct link_data *) _data;
+	struct mtp_udp_link *link = (struct mtp_udp_link *) _data;
 
-	snmp_mtp_activate(link->udp.session, link->udp.link_index);
-	mtp_link_up(link);
+	snmp_mtp_activate(link->session, link->link_index);
+	mtp_link_up(&link->base);
 }
 
-static int udp_link_reset(struct link_data *link)
+static int udp_link_reset(struct mtp_link *link)
 {
+	struct mtp_udp_link *ulnk;
+
+	ulnk = (struct mtp_udp_link *) link;
+
 	LOGP(DINP, LOGL_NOTICE, "Will restart SLTM transmission in %d seconds.\n",
-	     link->udp.reset_timeout);
-	snmp_mtp_deactivate(link->udp.session, link->udp.link_index);
+	     ulnk->reset_timeout);
+
+	snmp_mtp_deactivate(ulnk->session, ulnk->link_index);
 	mtp_link_down(link);
 
 	/* restart the link in 90 seconds... to force a timeout on the BSC */
 	link->link_activate.cb = do_start;
 	link->link_activate.data = link;
-	bsc_schedule_timer(&link->link_activate, link->udp.reset_timeout, 0);
+	bsc_schedule_timer(&link->link_activate, ulnk->reset_timeout, 0);
 	return 0;
 }
 
-static int udp_link_write(struct link_data *link, struct msgb *msg)
+static int udp_link_write(struct mtp_link *link, struct msgb *msg)
 {
+	struct mtp_udp_link *ulnk;
 	struct udp_data_hdr *hdr;
 
+	ulnk = (struct mtp_udp_link *) link;
+
 	hdr = (struct udp_data_hdr *) msgb_push(msg, sizeof(*hdr));
 	hdr->format_type = UDP_FORMAT_SIMPLE_UDP;
 	hdr->data_type = UDP_DATA_MSU_PRIO_0;
-	hdr->data_link_index = htons(link->udp.link_index);
+	hdr->data_link_index = htons(ulnk->link_index);
 	hdr->user_context = 0;
 	hdr->data_length = htonl(msgb_l2len(msg));
 
-	if (write_queue_enqueue(&link->udp.write_queue, msg) != 0) {
+	if (write_queue_enqueue(&ulnk->write_queue, msg) != 0) {
 		LOGP(DINP, LOGL_ERROR, "Failed to enqueue msg.\n");
 		msgb_free(msg);
 		return -1;
@@ -168,36 +176,36 @@
 	return 0;
 }
 
-static int udp_link_start(struct link_data *link)
+static int udp_link_start(struct mtp_link *link)
 {
 	LOGP(DINP, LOGL_NOTICE, "UDP input is ready.\n");
 	do_start(link);
 	return 0;
 }
 
-int link_udp_init(struct link_data *link, int src_port, const char *remote, int remote_port)
+int link_udp_init(struct mtp_udp_link *link, int src_port, const char *remote, int remote_port)
 {
 	struct sockaddr_in addr;
 	int fd;
 	int on;
 
-	write_queue_init(&link->udp.write_queue, 100);
+	write_queue_init(&link->write_queue, 100);
 
 	/* function table */
-	link->shutdown = udp_link_dummy;
-	link->clear_queue = udp_link_dummy;
+	link->base.shutdown = udp_link_dummy;
+	link->base.clear_queue = udp_link_dummy;
 
-	link->reset = udp_link_reset;
-	link->start = udp_link_start;
-	link->write = udp_link_write;
+	link->base.reset = udp_link_reset;
+	link->base.start = udp_link_start;
+	link->base.write = udp_link_write;
 
 	/* socket creation */
-	link->udp.write_queue.bfd.data = link;
-	link->udp.write_queue.bfd.when = BSC_FD_READ;
-	link->udp.write_queue.read_cb = udp_read_cb;
-	link->udp.write_queue.write_cb = udp_write_cb;
+	link->write_queue.bfd.data = link;
+	link->write_queue.bfd.when = BSC_FD_READ;
+	link->write_queue.read_cb = udp_read_cb;
+	link->write_queue.write_cb = udp_write_cb;
 
-	link->udp.write_queue.bfd.fd = fd = socket(AF_INET, SOCK_DGRAM, 0);
+	link->write_queue.bfd.fd = fd = socket(AF_INET, SOCK_DGRAM, 0);
 	if (fd < 0) {
 		LOGP(DINP, LOGL_ERROR, "Failed to create UDP socket.\n");
 		return -1;
@@ -218,12 +226,12 @@
 	}
 
 	/* now connect the socket to the remote */
-	memset(&link->udp.remote, 0, sizeof(link->udp.remote));
-	link->udp.remote.sin_family = AF_INET;
-	link->udp.remote.sin_port = htons(remote_port);
-	inet_aton(remote, &link->udp.remote.sin_addr);
+	memset(&link->remote, 0, sizeof(link->remote));
+	link->remote.sin_family = AF_INET;
+	link->remote.sin_port = htons(remote_port);
+	inet_aton(remote, &link->remote.sin_addr);
 
-	if (bsc_register_fd(&link->udp.write_queue.bfd) != 0) {
+	if (bsc_register_fd(&link->write_queue.bfd) != 0) {
 		LOGP(DINP, LOGL_ERROR, "Failed to register BFD.\n");
 		close(fd);
 		return -1;