workqueue: separate out drain_workqueue() from destroy_workqueue()

There are users which want to drain workqueues without destroying it.
Separate out drain functionality from destroy_workqueue() into
drain_workqueue() and make it accessible to workqueue users.

To guarantee forward-progress, only chain queueing is allowed while
drain is in progress.  If a new work item which isn't chained from the
running or pending work items is queued while draining is in progress,
WARN_ON_ONCE() is triggered.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: James Bottomley <James.Bottomley@hansenpartnership.com>

Change-Id: Ie976f83129e833e4ee0f2c4cebe997ed020cc691
[zwliew: adapt to previous linux-stable changes]
Signed-off-by: Zhao Wei Liew <zhaoweiliew@gmail.com>