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/osmo-bsc/osmo_bsc_filter.c b/openbsc/src/osmo-bsc/osmo_bsc_filter.c
index 608f525..596bfbd 100644
--- a/openbsc/src/osmo-bsc/osmo_bsc_filter.c
+++ b/openbsc/src/osmo-bsc/osmo_bsc_filter.c
@@ -78,11 +78,12 @@
 
 	switch (mi_type) {
 	case GSM_MI_TYPE_TMSI:
-		subscr = subscr_active_by_tmsi(conn->bts->network,
+		subscr = subscr_active_by_tmsi(conn->bts->network->subscr_group,
 					       tmsi_from_string(mi_string));
 		break;
 	case GSM_MI_TYPE_IMSI:
-		subscr = subscr_active_by_imsi(conn->bts->network, mi_string);
+		subscr = subscr_active_by_imsi(conn->bts->network->subscr_group,
+					       mi_string);
 		break;
 	default:
 		subscr = NULL;