io-wq: use helper for worker refcounting

Use io_worker_release() instead of hand coding it in io_worker_exit().

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/6f95f09d2cdbafcbb2e22ad0d1a2bc4d3962bf65.1634987320.git.asml.silence@gmail.com


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