io_uring: fix multi ctx cancellation

io_uring_try_cancel_requests() loops until there is nothing left to do
with the ring, however there might be several rings and they might have
dependencies between them, e.g. via poll requests.

Instead of cancelling rings one by one, try to cancel them all and only
then loop over if we still potenially some work to do.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/8d491fe02d8ac4c77ff38061cf86b9a827e8845c.1655684496.git.asml.silence@gmail.com


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