control_if: Close control connection socket/fd on read/write == 0

When read() or write() system calls return '0' on a stream socket,
it means that the connection has been closed ("EOF").  We must
accordingly close this socket and remove all related state.

Before this patch, every new CTRL connection would introduce a leak
of both some memory/state, as well as a file descriptor :(

Change-Id: I4fb70e5f123b37dece29f156c5f430c875e7cbaf
diff --git a/src/ctrl/control_if.c b/src/ctrl/control_if.c
index 17a012a..6293ff5 100644
--- a/src/ctrl/control_if.c
+++ b/src/ctrl/control_if.c
@@ -341,6 +341,7 @@
 		/* msg was already discarded. */
 		if (ret == 0) {
 			LOGP(DLCTRL, LOGL_INFO, "The control connection was closed\n");
+			control_close_conn(ccon);
 			ret = -EIO;
 		}
 		else
@@ -414,10 +415,17 @@
 
 static int control_write_cb(struct osmo_fd *bfd, struct msgb *msg)
 {
+	struct osmo_wqueue *queue;
+	struct ctrl_connection *ccon;
 	int rc;
 
+	queue = container_of(bfd, struct osmo_wqueue, bfd);
+	ccon = container_of(queue, struct ctrl_connection, write_queue);
+
 	rc = write(bfd->fd, msg->data, msg->len);
-	if (rc != msg->len)
+	if (rc == 0)
+		control_close_conn(ccon);
+	else if (rc != msg->len)
 		LOGP(DLCTRL, LOGL_ERROR, "Failed to write message to the control connection.\n");
 
 	return rc;