io_uring: end waiting before task cancel attempts

Get rid of TASK_UNINTERRUPTIBLE and waiting with finish_wait before
going for next iteration in __io_uring_task_cancel(), because
__io_uring_files_cancel() doesn't expect that sheduling is disallowed.

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