io-wq: make io_wqe_cancel_work() take a match handler

We want to use the cancel functionality for canceling based on not
just the work itself. Instead of matching on the work address
manually, allow a match handler to tell us if we found the right work
item or not.

No functional changes in this patch.

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