io_uring: use user visible tail in io_uring_poll()

We return POLLIN from io_uring_poll() depending on whether there are
CQEs for the userspace, and so we should use the user visible tail
pointer instead of a transient cached value.

Cc: stable@vger.kernel.org
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/228ffcbf30ba98856f66ffdb9a6a60ead1dd96c0.1674484266.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/io_uring.c

@@ -2886,7 +2886,7 @@ static __poll_t io_uring_poll(struct file *file, poll_table *wait)
 	 * pushes them to do the flush.
 	 */
 
-	if (io_cqring_events(ctx) || io_has_work(ctx))
+	if (__io_cqring_events_user(ctx) || io_has_work(ctx))
 		mask |= EPOLLIN | EPOLLRDNORM;
 
 	return mask;