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/libbsc/net_init.c b/openbsc/src/libbsc/net_init.c
index 1d37020..5bb8ae2 100644
--- a/openbsc/src/libbsc/net_init.c
+++ b/openbsc/src/libbsc/net_init.c
@@ -19,6 +19,7 @@
 
 #include <openbsc/gsm_data.h>
 #include <openbsc/osmo_msc_data.h>
+#include <openbsc/gsm_subscriber.h>
 
 struct gsm_network *gsm_network_init(uint16_t country_code, uint16_t network_code,
 				     int (*mncc_recv)(struct gsm_network *, struct msgb *))
@@ -35,10 +36,17 @@
 		return NULL;
 	}
 
+	net->subscr_group = talloc_zero(net, struct gsm_subscriber_group);
+	if (!net->subscr_group) {
+		talloc_free(net);
+		return NULL;
+	}
+
 	/* Init back pointer */
 	net->bsc_data->auto_off_timeout = -1;
 	net->bsc_data->network = net;
 	INIT_LLIST_HEAD(&net->bsc_data->mscs);
+	net->subscr_group->net = net;
 
 	net->country_code = country_code;
 	net->network_code = network_code;