io_uring: unify SQPOLL and user task cancellations

Merge io_uring_cancel_sqpoll() and __io_uring_cancel() as it's easier to
have a conditional ctx traverse inside than keeping them in sync.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/adfe24d6dad4a3883a40eee54352b8b65ac851bb.1623634181.git.asml.silence@gmail.com


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