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