traverse_visitbp: visit DMU_GROUPUSED_OBJECT before DMU_USERUSED_OBJECT

This is done to ensure that visited object IDs are always increasing.
Also, pass correct object ID to prefetch_dnode_metadata for
os_groupused_dnode.

Without this change we would hit an assert if traversal was paused on
a GROUPUSED object, which is unlikely but possible.

Apparently the same change was independently developed by Deplhix.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
MFC after: 10 days
Sponsored by: HybridCluster