io_uring: clean poll tw PF_EXITING handling

When we meet PF_EXITING in io_poll_check_events(), don't overcomplicate
the code with io_poll_mark_cancelled() but just return -ECANCELED and
the callers will deal with the rest.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/f0cc981af82a5b193658f8f44397eeb3bf838b7b.1650056133.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -5902,7 +5902,7 @@ static int io_poll_check_events(struct io_kiocb *req, bool locked)
 
 	/* req->task == current here, checking PF_EXITING is safe */
 	if (unlikely(req->task->flags & PF_EXITING))
-		io_poll_mark_cancelled(req);
+		return -ECANCELED;
 
 	do {
 		v = atomic_read(&req->poll_refs);