workqueue: rename wq->flush_mutex to wq->mutex

Currently pwq->flush_mutex protects many fields of a workqueue
including, especially, the pwqs list.  We're going to expand this
mutex to protect most of a workqueue and eventually replace pwq_lock,
which will make locking simpler and easier to understand.

Drop the "flush_" prefix in preparation.

This patch is pure rename.

tj: Rebased on top of the current dev branch.  Updated description.
    Use WQ: and WR: instead of Q: and QR: for synchronization labels.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>