io_uring: better ref handling in poll_remove_one

Instead of io_put_req() to drop not a final ref, use req_ref_put(),
which is slimmer and will also check the invariant.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/85b5774ce13ae55cc2e705abdc8cbafe1212f1bd.1617287883.git.asml.silence@gmail.com


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