workqueue: Make @flags handling consistent across set_work_data() and friends

- set_work_data() takes a separate @flags argument but just ORs it to @data.
  This is more confusing than helpful. Just take @data.

- Use the name @flags consistently and add the parameter to
  set_work_pool_and_{keep|clear}_pending(). This will be used by the planned
  disable/enable support.

No functional changes.

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