null_blk: do not del gendisk with lightnvm

The gendisk structure has not been initialized when using lightnvm.
Make sure to not delete it upon exit. Also make sure that we use the
appropriate disk_name at unregistration.

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