io_uring: add generic command payload type to struct io_kiocb

Each opcode generally has a command structure in io_kiocb which it can
use to store data associated with that request.

In preparation for having the core layer not know about what's inside
these fields, add a generic io_cmd_data type and put in the union as
well.

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