io_uring: hide caches sqes from drivers

There is now an io_uring private part of cmd async_data, move saved sqe
into it. Drivers are accessing it via struct io_uring_cmd::cmd.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/ecbe078dd57acefdbc4366d083327086c0879378.1743357121.git.asml.silence@gmail.com


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