io-wq: remove duplicated cancel code

Deduplicate cancellation parts, as many of them looks the same, as do
e.g.
- io_wqe_cancel_cb_work() and io_wqe_cancel_work()
- io_wq_worker_cancel() and io_work_cancel()

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>