workqueue: backport from LK 3.6.11 to LK 3.0

This backport allows 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

Change-Id: I67e5ce98327d1b448239432972023abea593a4a1
Signed-off-by: zwliew <zhaoweiliew@gmail.com>