msgb: remove smsh, llch, tlli and gmmh

This is a lot of GSM/GPRS specific stuff in struct msgb which we want
to avoid.  The 'control buffer' will replace them.
diff --git a/src/msgb.c b/src/msgb.c
index aea92d4..9117a0a 100644
--- a/src/msgb.c
+++ b/src/msgb.c
@@ -84,5 +84,5 @@
 	msg->lchan = NULL;
 	msg->l2h = NULL;
 	msg->l3h = NULL;
-	msg->smsh = NULL;
+	msg->l4h = NULL;
 }