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_rsl.c b/openbsc/src/abis_rsl.c
index b8990c7..0488b9d 100644
--- a/openbsc/src/abis_rsl.c
+++ b/openbsc/src/abis_rsl.c
@@ -293,7 +293,8 @@
 
 static struct msgb *rsl_msgb_alloc(void)
 {
-	return msgb_alloc_headroom(RSL_ALLOC_SIZE, RSL_ALLOC_HEADROOM);
+	return msgb_alloc_headroom(RSL_ALLOC_SIZE, RSL_ALLOC_HEADROOM,
+				   "RSL");
 }
 
 #define MACBLOCK_SIZE	23