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.

Christoph suggested last cycle that we 'simply' change quotaoff not to
allow deactivating quota entirely, but as these cleanups are to enable
one major change in behavior (deferred inode inactivation) I do not want
to add a second behavior change (quotaoff) as a dependency.

To be blunt: Additional cleanups are not in scope for this series.

v2: rebase to 5.13-rc4