PHP out of memory issue with Force Password Change

I am using the 7.1 branch of Password Policy and came across an issue with the Force Password Change admin feature. Our site has a large number of users (> 1 million records) and so on the Force Password Change page if a user role is chosen that has a large number of users in that role (obviously authenticated user would be large, but we also have age groups e.g. adult or teen and those are also large).

In the admin page form processing code, a query pulls a list of $uids that belong to the selected role. That array of uids is then passed as the value paramteter to a 'uid IN' condition on the password_policy_force_change table to set field force_change to true.

I found that the drupal db_update() call would throw a PHP out of memory error when the size of the array of $uids passed to the db_update() call was too large resulting in a WSOD.

I have written a small patch that chunks the $uids array into (arbitrary number) groups of 50,000 records to process at a time and iterates over each chunk when running the db_update() call. This resulted in the operation succeeding on the admin page w/o any PHP memory issue. Presumably it's Drupal core that when preparing the db_update() call probably runs into a memory limit when the $uids parameter is just too large.

This is a quick patch for us to get past this issue so feel free to alter and to apply to the 7.2 branch as we are not using that for now.