io_uring: add a warn_once for poll_find

io_poll_remove() expects poll_find() to search only for poll requests and
passes a flag for this. Just be a little bit extra cautious considering
lots of recent poll/cancellation changes and add a WARN_ON_ONCE checking
that we don't get an apoll'ed request.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/ec9a66f1e22f99dcd02288d4e42f3cc6bb357804.1655684496.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/poll.c

@@ -833,6 +833,11 @@ int io_poll_remove(struct io_kiocb *req, unsigned int issue_flags)
 	}
 
 found:
+	if (WARN_ON_ONCE(preq->opcode != IORING_OP_POLL_ADD)) {
+		ret = -EFAULT;
+		goto out;
+	}
+
 	if (poll_update->update_events || poll_update->update_user_data) {
 		/* only mask one event flags, keep behavior flags */
 		if (poll_update->update_events) {