a bug in quota check

[HenryShen]

\nova\db\sqlalchemy\api.py quota_reserve()

when decided whether to refresh the user_usages[resource], one rule is that if the last refresh was too long time ago, we need refresh user_usages[resource].

 elif max_age and (user_usages[resource].updated_at -
                              timeutils.utcnow()).seconds >= max_age:

using last update time minus current time result in a overflow ,so that the refresh action always be executed,in consideration of the max_age won't be a max number.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/121259

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/121259
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=f6d6c632f620411816979f04b7b3dda28681db18
Submitter: Jenkins
Branch: master

commit f6d6c632f620411816979f04b7b3dda28681db18
Author: Sean Dague <email address hidden>
Date: Fri Sep 12 18:51:29 2014 -0400

    correct inverted subtraction in quota check

    From a user reported bug: we were doing the subtraction backwards
    here, causing int overflow, causing all kinds of invalid. Instead
    actually subtract the smaller number from the bigger number.

    Change-Id: I7c099df5e31aa25b8be65c570e2e0b8a3e93c277
    Closes-Bug: #1363324