l2tp: fix refcount leak in l2tp_ip sockets

The l2tp_ip socket close handler does not update the module refcount
correctly which prevents module unload after the first bind() call on
an L2TPv3 IP encapulation socket.
Signed-off-by: NJames Chapman <jchapman@katalix.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/l2tp/l2tp_ip.c

@@ -232,7 +232,7 @@ static void l2tp_ip_close(struct sock *sk, long timeout)
 {
 	write_lock_bh(&l2tp_ip_lock);
 	hlist_del_init(&sk->sk_bind_node);
-	hlist_del_init(&sk->sk_node);
+	sk_del_node_init(sk);
 	write_unlock_bh(&l2tp_ip_lock);
 	sk_common_release(sk);
 }