gtp_{ggsn,mme}: Allocate contexts under struct sgsn_instance

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

Change-Id: Ie65d59632a368c6957c33dca64e856ace792b2c6
diff --git a/src/sgsn/gprs_sm.c b/src/sgsn/gprs_sm.c
index 157e279..184350d 100644
--- a/src/sgsn/gprs_sm.c
+++ b/src/sgsn/gprs_sm.c
@@ -378,7 +378,7 @@
 		goto reject_due_failure;
 	}
 
-	ggsn = sgsn_ggsn_ctx_alloc(UINT32_MAX);
+	ggsn = sgsn_ggsn_ctx_alloc(sgsn, UINT32_MAX);
 	if (!ggsn) {
 		LOGMMCTXP(LOGL_ERROR, lookup->mmctx, "Failed to create ggsn.\n");
 		goto reject_due_failure;