tcp: md5: correct a RCU lockdep splat

commit a8afca0 (tcp: md5: protects md5sig_info with RCU) added a
lockdep splat in tcp_md5_do_lookup() in case a timer fires a tcp
retransmit.

At this point, socket lock is owned by the sofirq handler, not the user,
so we should adjust a bit the lockdep condition, as we dont hold
rcu_read_lock().

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Reported-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_ipv4.c

@@ -927,7 +927,8 @@ struct tcp_md5sig_key *tcp_md5_do_lookup(struct sock *sk,
 
 	/* caller either holds rcu_read_lock() or socket lock */
 	md5sig = rcu_dereference_check(tp->md5sig_info,
-				       sock_owned_by_user(sk));
+				       sock_owned_by_user(sk) ||
+				       lockdep_is_held(&sk->sk_lock.slock));
 	if (!md5sig)
 		return NULL;
 #if IS_ENABLED(CONFIG_IPV6)