[paging] Introduce a variable containing the free paging slots

Start with a large number of available slots. It is guranteed
that we will - at some point - get a paging load and will properly
update the counter and keep it updated.

diff --git a/src/paging.c b/src/paging.c
index 4826690..55f6d81 100644
--- a/src/paging.c
+++ b/src/paging.c
@@ -134,6 +134,9 @@
 	INIT_LLIST_HEAD(&bts->paging.pending_requests);
 	bts->paging.paging_timer.cb = paging_handle_pending_requests;
 	bts->paging.paging_timer.data = &bts->paging;
+
+	/* Large number, until we get a proper message */
+	bts->paging.available_slots = 0xffff;
 }
 
 static int paging_pending_request(struct gsm_bts_paging_state *bts,
@@ -181,3 +184,8 @@
 		}
 	}
 }
+
+void paging_update_buffer_space(struct gsm_bts *bts, u_int16_t free_slots)
+{
+	bts->paging.available_slots = free_slots;
+}