l2tp: avoid use-after-free caused by l2tp_ip_backlog_recv

l2tp_ip_backlog_recv may not return -1 if the packet gets dropped.
The return value is passed up to ip_local_deliver_finish, which treats
negative values as an IP protocol number for resubmission.
Signed-off-by: NPaul Hüber <phueber@kernsp.in>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/l2tp/l2tp_ip.c

@@ -388,7 +388,7 @@ static int l2tp_ip_backlog_recv(struct sock *sk, struct sk_buff *skb)
 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