lightnvm: make geometry structures 2.0 ready

Prepare for the 2.0 revision by adapting the geometry
structures to coexist with the 1.2 revision.

Signed-off-by: Matias Bjørling <m@bjorling.me>
Reviewed-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>