lightnvm: cleanup nvm transformation functions

Going from target specific ppa addresses to device was accomplished by
first converting target to generic ppa addresses and generic to device
addresses. The conversion was either open-coded or used the built-in
nvm_trans_* and nvm_map_* functions for conversion. Simplify the
interface and cleanup the calls to provide clean functions that now
either take a list of ppas or a nvm_rq, and is exposed through:

 void nvm_ppa_* - target to/from device with a list of PPAs,
 void nvm_rq_* - target to/from device with a nvm_rq.

Signed-off-by: Matias Bjørling <matias@cnexlabs.com>
Signed-off-by: Jens Axboe <axboe@fb.com>