io_uring: carve io_poll_check_events fast path

The fast path in io_poll_check_events() is when we have only one
(i.e. master) reference. Move all verification, cancellations
checks, edge case handling and so on under a common if.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/8c21c5d5e027e32dc553705e88796dec79ff6f93.1669821213.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>