use named variant when allocating msgb's

when we generate a talloc report (SIGUSR1), we can now see which system
allocated a given msgb, this helps memory leak debugging
diff --git a/openbsc/src/gsm_04_08.c b/openbsc/src/gsm_04_08.c
index c081b45..56db654 100644
--- a/openbsc/src/gsm_04_08.c
+++ b/openbsc/src/gsm_04_08.c
@@ -975,7 +975,8 @@
 
 struct msgb *gsm48_msgb_alloc(void)
 {
-	return msgb_alloc_headroom(GSM48_ALLOC_SIZE, GSM48_ALLOC_HEADROOM);
+	return msgb_alloc_headroom(GSM48_ALLOC_SIZE, GSM48_ALLOC_HEADROOM,
+				   "GSM 04.08");
 }
 
 int gsm48_sendmsg(struct msgb *msg)
@@ -1856,7 +1857,7 @@
 
 	mncc->msg_type = msg_type;
 	
-	msg = msgb_alloc(sizeof(struct gsm_mncc));
+	msg = msgb_alloc(sizeof(struct gsm_mncc), "MNCC");
 	if (!msg)
 		return -ENOMEM;
 	memcpy(msg->data, mncc, sizeof(struct gsm_mncc));