lightnvm: add media manager mark_blk helper

Expose media manager mark_blk() to targets, as done for the rest of the
media manager callback functions.

Signed-off-by: Javier González <javier@cnexlabs.com>
Updated description
Signed-off-by: Matias Bjørling <m@bjorling.me>

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