Illumos 4504 traverse_visitbp: visit group before user

4504 traverse_visitbp: visit DMU_GROUPUSED_OBJECT before DMU_USERUSED_OBJECT

Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>

References:

https://illumos.org/issues/4504
http://code.delphix.com/illumos-4504
http://svnweb.freebsd.org/base?view=revision&revision=260812

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Closes #2079