l2tp: avoid use-after-free caused by l2tp_ip_backlog_recv
am: 837786cbbb

Change-Id: I50cc6eec703138b7b7e9163ae7424f541947be1c
diff --git a/net/l2tp/l2tp_ip.c b/net/l2tp/l2tp_ip.c
index c0f0750..ff750bb 100644
--- a/net/l2tp/l2tp_ip.c
+++ b/net/l2tp/l2tp_ip.c
@@ -388,7 +388,7 @@
 drop:
 	IP_INC_STATS(sock_net(sk), IPSTATS_MIB_INDISCARDS);
 	kfree_skb(skb);
-	return -1;
+	return 0;
 }
 
 /* Userspace will call sendmsg() on the tunnel socket to send L2TP