io_uring/cmd: add iovec cache for commands

Add iou_vec to commands and wire caching for it, but don't expose it to
users just yet. We need the vec cleared on initial alloc, but since
we can't place it at the beginning at the moment, zero the entire
async_data. It's cached, and the performance effects only the initial
allocation, and it might be not a bad idea since we're exposing those
bits to outside drivers.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/c0f2145b75791bc6106eb4e72add2cf6a2c72a7a.1742579999.git.asml.silence@gmail.com


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