lightnvm: make 1.2 data structures explicit

Make the 1.2 data structures explicit, so it will be easy to identify
the 2.0 data structures. Also fix the order of which the nvme_nvm_*
are declared, such that they follow the nvme_nvm_command order.

Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>