[nat] Lookup by BSC Connection otherwise the point of reassigning the is defeated

When sending a MSG to the MSC try to find the to be used "src" reference
by comparing the reference on the BSC and the BSC connection. Only this
tuple needs to be unique.
Actually only when looking at the SRC REF we need to compare the BSC as the
dest reference should be unique but we are just making the check a bit stronger
to make it look symmetric.
diff --git a/openbsc/src/nat/bsc_sccp.c b/openbsc/src/nat/bsc_sccp.c
index e84ffe9..58302cc 100644
--- a/openbsc/src/nat/bsc_sccp.c
+++ b/openbsc/src/nat/bsc_sccp.c
@@ -204,11 +204,14 @@
  */
 struct sccp_connections *patch_sccp_src_ref_to_msc(struct msgb *msg,
 						   struct bsc_nat_parsed *parsed,
-						   struct bsc_nat *nat)
+						   struct bsc_connection *bsc)
 {
 	struct sccp_connections *conn;
 
-	llist_for_each_entry(conn, &nat->sccp_connections, list_entry) {
+	llist_for_each_entry(conn, &bsc->nat->sccp_connections, list_entry) {
+		if (conn->bsc != bsc)
+			continue;
+
 		if (parsed->src_local_ref) {
 			if (equal(parsed->src_local_ref, &conn->real_ref)) {
 				*parsed->src_local_ref = conn->patched_ref;