workqueue: Implement disable/enable for (delayed) work items

While (delayed) work items could be flushed and canceled, there was no way
to prevent them from being queued in the future. While this didn't lead to
functional deficiencies, it sometimes required a bit more effort from the
workqueue users to e.g. sequence shutdown steps with more care.

Workqueue is currently in the process of replacing tasklet which does
support disabling and enabling. The feature is used relatively widely to,
for example, temporarily suppress main path while a control plane operation
(reset or config change) is in progress.

To enable easy conversion of tasklet users and as it seems like an inherent
useful feature, this patch implements disabling and enabling of work items.

- A work item carries 16bit disable count in work->data while not queued.
  The access to the count is synchronized by the PENDING bit like all other
  parts of work->data.

- If the count is non-zero, the work item cannot be queued. Any attempt to
  queue the work item fails and returns %false.

- disable_work[_sync](), enable_work(), disable_delayed_work[_sync]() and
  enable_delayed_work() are added.

v3: enable_work() was using local_irq_enable() instead of
    local_irq_restore() to undo IRQ-disable by work_grab_pending(). This is
    awkward now and will become incorrect as enable_work() will later be
    used from IRQ context too. (Lai)

v2: Lai noticed that queue_work_node() wasn't checking the disable count.
    Fixed. queue_rcu_work() is updated to trigger warning if the inner work
    item is disabled.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <jiangshanlai@gmail.com>