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/abis_nm.c b/openbsc/src/abis_nm.c
index 6b1fc54..4a73930 100644
--- a/openbsc/src/abis_nm.c
+++ b/openbsc/src/abis_nm.c
@@ -423,7 +423,8 @@
 
 static struct msgb *nm_msgb_alloc(void)
 {
-	return msgb_alloc_headroom(OM_ALLOC_SIZE, OM_HEADROOM_SIZE);
+	return msgb_alloc_headroom(OM_ALLOC_SIZE, OM_HEADROOM_SIZE,
+				   "OML");
 }
 
 /* Send a OML NM Message from BSC to BTS */