vsock: set EPOLLERR on non-empty error queue

If socket's error queue is not empty, EPOLLERR must be set. Otherwise,
reader of error queue won't detect data in it using EPOLLERR bit.
Currently for AF_VSOCK this is actual only with MSG_ZEROCOPY, as this
feature is the only user of an error queue of the socket.
Signed-off-by: Arseniy Krasnov <avkrasnov@salutedevices.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/vmw_vsock/af_vsock.c

@@ -1030,7 +1030,7 @@ static __poll_t vsock_poll(struct file *file, struct socket *sock,
 	poll_wait(file, sk_sleep(sk), wait);
 	mask = 0;
 
-	if (sk->sk_err)
+	if (sk->sk_err || !skb_queue_empty_lockless(&sk->sk_error_queue))
 		/* Signify that there has been an error on this socket. */
 		mask |= EPOLLERR;