lightnvm: introduce helpers for generic ops in rrpc

Avoid calling media manager and device-specific operations directly from
rrpc. Create helper functions on lightnvm's core instead.

Signed-off-by: Javier González <javier@cnexlabs.com>

Made it work with null_blk as well.
Signed-off-by: Matias Bjørling <m@bjorling.me>

Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>