null_blk: add lightnvm null_blk device to the nullb_list

After register null_blk devices into lightnvm, we forget
to add these devices to the the nullb_list, makes them
invisible to the null_blk driver.

Signed-off-by: Wenwei Tao <ww.tao0320@gmail.com>
Fixes: a514379b ("null_blk: oops when initializing without lightnvm")
Signed-off-by: Jens Axboe <axboe@fb.com>