io_uring/cmd: remove struct io_uring_cmd_data

There are no more users of struct io_uring_cmd_data and its op_data
field. Remove it to shave 8 bytes from struct io_async_cmd and eliminate
a store and load for every uring_cmd.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Acked-by: David Sterba <dsterba@suse.com>
Link: https://lore.kernel.org/r/20250708202212.2851548-5-csander@purestorage.com


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