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_11.c b/openbsc/src/gsm_04_11.c
index 571da4c..1b622b1 100644
--- a/openbsc/src/gsm_04_11.c
+++ b/openbsc/src/gsm_04_11.c
@@ -51,7 +51,8 @@
 
 struct msgb *gsm411_msgb_alloc(void)
 {
-	return msgb_alloc_headroom(GSM411_ALLOC_SIZE, GSM411_ALLOC_HEADROOM);
+	return msgb_alloc_headroom(GSM411_ALLOC_SIZE, GSM411_ALLOC_HEADROOM,
+				   "GSM 04.11");
 }
 
 int gsm0411_sendmsg(struct msgb *msg)