workqueue: Introduce work_cancel_flags

The cancel path used bool @is_dwork to distinguish canceling a regular work
and a delayed one. The planned disable/enable support will need passing
around another flag in the code path. As passing them around with bools will
be confusing, let's introduce named flags to pass around in the cancel path.

WORK_CANCEL_DELAYED replaces @is_dwork. No functional changes.

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