[msc/nat] It is better to use msgb_free to free the msgb

msgb_free is currently calling talloc_free but this might
change in the future and then this code would break..
diff --git a/openbsc/src/nat/bsc_nat.c b/openbsc/src/nat/bsc_nat.c
index 8e31188..6ce78be 100644
--- a/openbsc/src/nat/bsc_nat.c
+++ b/openbsc/src/nat/bsc_nat.c
@@ -401,7 +401,7 @@
 	else if (hh->proto == IPAC_PROTO_SCCP)
 		forward_sccp_to_bts(msg);
 
-	talloc_free(msg);
+	msgb_free(msg);
 	return 0;
 }
 
@@ -568,7 +568,7 @@
 	/* FIXME: Currently no PONG is sent to the BSC */
 	/* FIXME: Currently no ID ACK is sent to the BSC */
 	forward_sccp_to_msc(bfd, msg);
-	talloc_free(msg);
+	msgb_free(msg);
 
 	return 0;
 }