io_uring/uring_cmd: assign ioucmd->cmd at async prep time

Rather than check this in the fast path issue, it makes more sense to
just assign the copy of the data when we're setting it up anyway. This
makes the code a bit cleaner, and removes the need for this check in
the issue path.

Reviewed-by: Gabriel Krisman Bertazi <krisman@suse.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>