Illumos 4504

4504 traverse_visitbp: visit DMU_GROUPUSED_OBJECT before DMU_USERUSED_OBJECT

References:
  https://illumos.org/issues/4504
  http://code.delphix.com/illumos-4504
  http://svnweb.freebsd.org/base?view=revision&revision=260812

Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>