io_uring: add infrastructure around io_uring_cmd_sqe

io_uring: add infrastructure around io_uring_cmd_sqe

Define an io_uring_cmd_sqe struct that passthrough commands can use,
and define an array that has offset information for the two members
that we care about (user_data and personality). Then we can init the
two command types in basically the same way, just reading the user_data
and personality at the defined offsets for the command type.

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