workqueue/hotplug: simplify workqueue_offline_cpu()

Since the recent cpu/hotplug refactoring, workqueue_offline_cpu() is
guaranteed to run on the local cpu which is going offline.

This also fixes the following deadlock by removing work item
scheduling and flushing from CPU hotplug path.

 http://lkml.kernel.org/r/1504764252-29091-1-git-send-email-prsood@codeaurora.org



tj: Description update.

Change-Id: I3b2b9f110591abea36027503eabce72465373926
Signed-off-by: Lai Jiangshan <jiangshanlai@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Git-commit: e8b3f8db7aad99fcc5234fc5b89984ff6620de3d
Git-Repo: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/


Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>