io_uring: clarify io_req_task_cancel() locking

It's too easy to forget and misjudge about synchronisation in
io_req_task_cancel(), add a comment clarifying it.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/71099083835f983a1fd73d5a3da6391924da8300.1629920396.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>