io_uring: deduplicate cancellation code

IORING_OP_ASYNC_CANCEL and IORING_OP_LINK_TIMEOUT have enough of
overlap, so extract a helper for request cancellation and use in both.
Also, removes some amount of ugliness because of success_ret.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/900122b588e65b637e71bfec80a260726c6a54d6.1628981736.git.asml.silence@gmail.com


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