Btrfs: cleanup orphan reservation if truncate fails

I noticed we were getting lots of warnings with xfstest 83 because we have
reservations outstanding.  This is because we moved the orphan add outside
of the truncate, but we don't actually cleanup our reservation if something
fails.  This fixes the problem and I no longer see warnings.  Thanks,

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