Btrfs: cleanup orphaned root orphan item

I hit a weird problem were my root item had been deleted but the orphan item had
not.  This isn't necessarily a problem, but it keeps the file system from being
mounted.  To fix this we just need to axe the orphan item if we can't find the
fs root when we're putting them altogether.  With this patch I was able to
successfully mount my file system.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>