lightnvm: pblk: refactor bad block identification

In preparation for the OCSSD 2.0 spec. bad block identification,
refactor the current code to generalize bad block get/set functions and
structures.

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