btrfs: check squota parent usage on membership change

btrfs: check squota parent usage on membership change

We could have detected the quick inherit bug more directly if we had
an extra warning about squota hierarchy consistency while modifying the
hierarchy. In squotas, the parent usage always simply adds up to the sum of
its children, so we can just check for that when changing membership and
detect more accounting bugs.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>