io_uring/cmd: give inline space in request to cmds

Some io_uring commands can use some inline space in io_kiocb. We have 32
bytes in struct io_uring_cmd, expose it.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/7ca779a61ee5e166e535d70df9c7f07b15d8a0ce.1726072086.git.asml.silence@gmail.com


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