af_unix: Use skb_queue_len_lockless() in sk_diag_show_rqlen().

We can dump the socket queue length via UNIX_DIAG by specifying
UDIAG_SHOW_RQLEN.

If sk->sk_state is TCP_LISTEN, we return the recv queue length,
but here we do not hold recvq lock.

Let's use skb_queue_len_lockless() in sk_diag_show_rqlen().

Fixes: c9da99e6 ("unix_diag: Fixup RQLEN extension report")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/unix/diag.c

@@ -104,7 +104,7 @@ static int sk_diag_show_rqlen(struct sock *sk, struct sk_buff *nlskb)
 	struct unix_diag_rqlen rql;
 
 	if (READ_ONCE(sk->sk_state) == TCP_LISTEN) {
-		rql.udiag_rqueue = sk->sk_receive_queue.qlen;
+		rql.udiag_rqueue = skb_queue_len_lockless(&sk->sk_receive_queue);
 		rql.udiag_wqueue = sk->sk_max_ack_backlog;
 	} else {
 		rql.udiag_rqueue = (u32) unix_inq_len(sk);