commit 3681c85d upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>