libgb: don't call directly into GMM / LLC layer

Instead of direct function calls to individual functions, we now
generate primitives (osmo_prim) and send them to one
application-provided function "bssgp_prim_cb()"
diff --git a/openbsc/src/gprs/gprs_llc.c b/openbsc/src/gprs/gprs_llc.c
index c1327b6..be5fe52 100644
--- a/openbsc/src/gprs/gprs_llc.c
+++ b/openbsc/src/gprs/gprs_llc.c
@@ -48,7 +48,6 @@
 	dup.drx_parms = mmctx->drx_parms;
 	dup.ms_ra_cap.len = mmctx->ms_radio_access_capa.len;
 	dup.ms_ra_cap.v = mmctx->ms_radio_access_capa.buf;
-	dup.pdu_lifetime = 1000; /* centi-seconds */
 	memcpy(&dup.qos_profile, qos_profile_default,
 		sizeof(qos_profile_default));
 
@@ -328,7 +327,7 @@
 	/* Identifiers passed down: (BVCI, NSEI) */
 
 	/* Send BSSGP-DL-UNITDATA.req */
-	return _bssgp_tx_dl_ud(msg, 1000, NULL);
+	return _bssgp_tx_dl_ud(msg, NULL);
 }
 
 /* Send XID response to LLE */