io_uring: always go for cancellation spin on exec

Always try to do cancellation in __io_uring_task_cancel() at least once,
so it actually goes and cleans its sqpoll tasks (i.e. via
io_sqpoll_cancel_sync()), otherwise sqpoll task may submit new requests
after cancellation and it's racy for many reasons.

Fixes: 521d6a73

 ("io_uring: cancel sqpoll via task_work")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/0a21bd6d794bb1629bc906dd57a57b2c2985a8ac.1616839147.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>