Btrfs send/scrub/defrag/qgroup need to walk backrefs,this test
is to make sure iterating backrefs with ulist is working and don't
cause a kernel panic here.
Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>
Wang Shilong