make sure subscr->net is always set

since a subscriber is an element of the gsm_network, we have to ensure
subscr->net is always set correctly.  We do this by using gsm_network
as an argument to all functions that resolve or create a subscriber.
diff --git a/openbsc/src/gsm_04_11.c b/openbsc/src/gsm_04_11.c
index 1b622b1..8db402b 100644
--- a/openbsc/src/gsm_04_11.c
+++ b/openbsc/src/gsm_04_11.c
@@ -154,6 +154,7 @@
 /* process an incoming TPDU (called from RP-DATA) */
 static int gsm340_rx_tpdu(struct msgb *msg)
 {
+	struct gsm_bts *bts = msg->lchan->ts->trx->bts;
 	u_int8_t *smsp = msgb_sms(msg);
 	struct sms_submit *sms;
 	struct gsm_sms *gsms;
@@ -257,7 +258,7 @@
 	/* FIXME: sender refcount */
 
 	/* determine gsms->receiver based on dialled number */
-	gsms->receiver = subscr_get_by_extension(sms->dest_addr);
+	gsms->receiver = subscr_get_by_extension(bts->network, sms->dest_addr);
 	if (!gsms->receiver) {
 		rc = 1; /* cause 1: unknown subscriber */
 		goto out;