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/sgsn_vty.c b/src/sgsn/sgsn_vty.c
index 8e94041..d8ea420 100644
--- a/src/sgsn/sgsn_vty.c
+++ b/src/sgsn/sgsn_vty.c
@@ -723,7 +723,7 @@
 {
 	struct sgsn_pdp_ctx *pdp;
 
-	llist_for_each_entry(pdp, &sgsn_pdp_ctxts, g_list)
+	llist_for_each_entry(pdp, &sgsn->pdp_list, g_list)
 		vty_dump_pdp(vty, "", pdp);
 
 	return CMD_SUCCESS;