xfs: clean up quotaoff inode walks

This series cleans up the inode walk that quotaoff does to detach dquots
from the current set of incore inodes.  Once we've cleared away all the
baggage that was implemented for "no tag" walks (since this is the only
caller that doesn't use tags), we can speed up quotaoff by making it
detach more aggressively.  This gets us into shape for further inode
walk cleanups in the next series.

v2: rebase to 5.13