Avoid calling media manager and device-specific operations directly from
rrpc. Create helper functions on lightnvm's core instead.
Signed-off-by: NJavier González <javier@cnexlabs.com>

Made it work with null_blk as well.
Signed-off-by: NMatias Bjørling <m@bjorling.me>
Signed-off-by: NMatias Bjørling <m@bjorling.me>
Signed-off-by: NJens Axboe <axboe@fb.com>