Move global pdp_list inside struct sgsn_instance

This way pdp contexts are managed by the lifcycle of the main global struct
sgsn_instance automatically.

Change-Id: I725218fd54adcc68dceded5eb43675f25771bb96
diff --git a/src/sgsn/gprs_sgsn.c b/src/sgsn/gprs_sgsn.c
index 9d2ed7d..40efd4a 100644
--- a/src/sgsn/gprs_sgsn.c
+++ b/src/sgsn/gprs_sgsn.c
@@ -704,6 +704,7 @@
 	INIT_LLIST_HEAD(&inst->apn_list);
 	INIT_LLIST_HEAD(&inst->ggsn_list);
 	INIT_LLIST_HEAD(&inst->mme_list);
+	INIT_LLIST_HEAD(&inst->pdp_list);
 
 	osmo_timer_setup(&inst->llme_timer, sgsn_llme_check_cb, NULL);
 	osmo_timer_schedule(&inst->llme_timer, GPRS_LLME_CHECK_TICK, 0);