Btrfs: don't bother autodefragging if our root is going away

We can end up with inodes on the auto defrag list that exist on roots that are
going to be deleted.  This is extra work we don't need to do, so just bail if
our root has 0 root refs.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>