nvme: use helpers to access io_uring cmd space

Command implementations shouldn't be directly looking into io_uring_cmd
to carve free space. Use an io_uring helper, which will also do build
time size sanitisation.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Kanchan Joshi <joshi.k@samsung.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Anuj Gupta <anuj20.g@samsung.com>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Keith Busch <kbusch@kernel.org>