udp: Create a SNMP session per UDP

We do not have the multiple callbacks from SNMP under control
and we can only save the last request if the SNMP Session is
inside the link. This is mostly a workaround for Net-SNMP and
the missing documentation on the async functionality.
diff --git a/src/links.c b/src/links.c
index 086b95c..3977daf 100644
--- a/src/links.c
+++ b/src/links.c
@@ -116,7 +116,7 @@
 
 	LOGP(DINP, LOGL_NOTICE, "Using UDP MTP mode.\n");
 
-	if (link_global_init(&bsc->udp_data, bsc->udp_ip, bsc->src_port) != 0)
+	if (link_global_init(&bsc->udp_data, bsc->src_port) != 0)
 		return -1;
 
 
@@ -140,7 +140,7 @@
 		 * SLTM and it begins a reset. Then we will take it up
 		 * again and do the usual business.
 		 */
-		snmp_mtp_deactivate(lnk->data->session,
+		snmp_mtp_deactivate(lnk->session,
 				    lnk->link_index);
 		bsc->start_timer.cb = start_rest;
 		bsc->start_timer.data = &bsc;