gtp-kernel: shut down kernel GTP device in apn_down()

When we take the APN down, we should also take the GTP device down.

Change-Id: Idd250dd454a1603834d388884a24a63e044fdd7b
diff --git a/ggsn/ggsn.c b/ggsn/ggsn.c
index 6aa4210..1d4e592 100644
--- a/ggsn/ggsn.c
+++ b/ggsn/ggsn.c
@@ -130,6 +130,7 @@
 		tun_free(apn->tun.tun);
 		apn->tun.tun = NULL;
 	}
+	gtp_kernel_stop(apn->tun.cfg.dev_name);
 
 	if (apn->v4.pool) {
 		LOGPAPN(LOGL_INFO, apn, "Releasing IPv4 pool\n");