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: Ibc7095a0f74b477c85030512ec2e3b20b6ec1c32
Signed-off-by: Lai Jiangshan <jiangshanlai@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Git-commit: e8b3f8db7aad99fcc5234fc5b89984ff6620de3d
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Prateek Sood <prsood@codeaurora.org>