nat: Introduce reject cause to bsc_nat_acc_lst_entry

The filtering architecture already allowed to specify a reject
reason but this has not been used for the access-lists. Extend
the access-list to include a reject reason and extend the test
case to honor it.
diff --git a/openbsc/src/osmo-bsc_nat/bsc_nat_utils.c b/openbsc/src/osmo-bsc_nat/bsc_nat_utils.c
index bc8c4c1..236a0fb 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_nat_utils.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_nat_utils.c
@@ -468,6 +468,8 @@
 	if (!entry)
 		return NULL;
 
+	entry->cm_reject_cause = GSM48_REJECT_PLMN_NOT_ALLOWED;
+	entry->lu_reject_cause = GSM48_REJECT_PLMN_NOT_ALLOWED;
 	llist_add_tail(&entry->list, &lst->fltr_list);
 	return entry;
 }