msc: Add and use gsm_subscriber_group

Currently every subcriber object directly refers to the gsm_network
which contains a flag shared by every related subscriber
(keep_subscr). This adds a dependency on gsm_network even if only the
function defined in gsm_subscriber_base.c are used.

This patch adds a new struct gsm_subscriber_group which contains the
keep_subscr flag and a back reference to the network object. The
latter is not dereferenced in gsm_subscriber_base.c, so it can safely
be set to NULL when only that part of the gsm_subscriber API is being
used. It also changes that API to use gsm_subscriber_group instead of
gsm_network parameters.

Since there are some places where a pointer to the gsm_network is
needed but where only a gsm_subscriber is available, a 'net' back
pointer is added to the group struct, too. Nevertheless subscr group
and network could be separated completely, but this is not the topic
of this commit.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/src/libmsc/sms_queue.c b/openbsc/src/libmsc/sms_queue.c
index 536bfdd..3ed8b56 100644
--- a/openbsc/src/libmsc/sms_queue.c
+++ b/openbsc/src/libmsc/sms_queue.c
@@ -135,7 +135,7 @@
 
 	pending->resend = 1;
 
-	smsq = pending->subscr->net->sms_queue;
+	smsq = pending->subscr->group->net->sms_queue;
 	if (osmo_timer_pending(&smsq->resend_pending))
 		return;
 
@@ -149,7 +149,7 @@
 	LOGP(DLSMS, LOGL_NOTICE, "Sending SMS %llu failed %d times.\n",
 	     pending->sms_id, pending->failed_attempts);
 
-	smsq = pending->subscr->net->sms_queue;
+	smsq = pending->subscr->group->net->sms_queue;
 	if (++pending->failed_attempts < smsq->max_fail)
 		return sms_pending_resend(pending);
 
@@ -285,7 +285,7 @@
  */
 static void sms_send_next(struct gsm_subscriber *subscr)
 {
-	struct gsm_sms_queue *smsq = subscr->net->sms_queue;
+	struct gsm_sms_queue *smsq = subscr->group->net->sms_queue;
 	struct gsm_sms_pending *pending;
 	struct gsm_sms *sms;
 
@@ -316,7 +316,7 @@
 
 no_pending_sms:
 	/* Try to send the SMS to avoid the queue being stuck */
-	sms_submit_pending(subscr->net->sms_queue);
+	sms_submit_pending(subscr->group->net->sms_queue);
 }
 
 /*