lightnvm: transform target get/set bad block

Since targets are given a virtual target device, it is necessary to
translate all communication between targets and the backend device.
Implement the translation layer for get/set bad block table.

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