Revert "workqueue: restructure pool / pool_workqueue iterations in freeze/thaw functions"

This reverts commit 64f648a6bbc2789b65cc845a95cbc101074cc3c1.
Revert "workqueue: introduce for_each_pool()"

This reverts commit 44c1a67739aa6bc49b115995e28c6a7fe09565bf.