Targets can update a block state when having a reference to an
in-memory virtual block. In the case that a target does not keep the
block metadata in memory, it does not have a way to update this
structure.

Therefore, expose gennvm_mark_blk() through the media managers
->mark_blk() callback and let targets update the state structure through
this callback.
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>