io_uring: batch cancel in io_uring_cancel_files()

Instead of waiting for each request one by one, first try to cancel all
of them in a batched manner, and then go over inflight_list/etc to reap
leftovers.

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