lightnvm: make address conversions depend on generic device

On address conversions, use the generic device, instead of the target
device. This allows to use conversions outside of the target's realm.

Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>