l2tp: prevent lockdep issue in l2tp_tunnel_register()

lockdep complains with the following lock/unlock sequence:

     lock_sock(sk);
     write_lock_bh(&sk->sk_callback_lock);
[1]  release_sock(sk);
[2]  write_unlock_bh(&sk->sk_callback_lock);

We need to swap [1] and [2] to fix this issue.

Fixes: 0b2c5972 ("l2tp: close all race conditions in l2tp_tunnel_register()")
Reported-by: syzbot+bbd35b345c7cab0d9a08@syzkaller.appspotmail.com
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/netdev/20230114030137.672706-1-xiyou.wangcong@gmail.com/T/#m1164ff20628671b0f326a24cb106ab3239c70ce3
Cc: Cong Wang <cong.wang@bytedance.com>
Cc: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_core.c

@@ -1483,10 +1483,8 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
 	lock_sock(sk);
 	write_lock_bh(&sk->sk_callback_lock);
 	ret = l2tp_validate_socket(sk, net, tunnel->encap);
-	if (ret < 0) {
-		release_sock(sk);
+	if (ret < 0)
 		goto err_inval_sock;
-	}
 	rcu_assign_sk_user_data(sk, tunnel);
 	write_unlock_bh(&sk->sk_callback_lock);
 
@@ -1523,6 +1521,7 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
 
 err_inval_sock:
 	write_unlock_bh(&sk->sk_callback_lock);
+	release_sock(sk);
 
 	if (tunnel->fd < 0)
 		sock_release(sock);