workqueue: backport from Linux 3.6.11

to get the following patches by Tejun Heo that improves the latency of cpu
hotplug events.
workqueue: simplify CPU hotplug code
workqueue: remove CPU offline trustee
workqueue: don't butcher idle workers on an offline CPU
workqueue: reimplement CPU online rebinding to handle idle workers
https://wiki.linaro.org/WorkingGroups/PowerManagement/Doc/Hotplug#Workqueue