Btrfs: make backref walking code handle skinny metadata

Btrfs: make backref walking code handle skinny metadata

I missed fixing the backref stuff when I introduced the skinny metadata.  If you
try and do things like snapshot aware defrag with skinny metadata you are going
to see tons of warnings related to the backref count being less than 0.  This is
because the delayed refs will be found for stuff just fine, but it won't find
the skinny metadata extent refs.  With this patch I'm not seeing warnings
anymore.  Thanks,

Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>