btrfs: quota rescan helpers

Many btrfs tests explicitly trigger quota rescan. This is not a
meaningful operation for simple quotas, so we wrap it in a helper that
doesn't blow up quite so badly and lets us run those tests where the
rescan is a qgroup detail.

Signed-off-by: Boris Burkov <boris@bur.io>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>