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
6d03ca5789