workqueue: allow more off-queue flag space

When a work item is off-queue, its work->data contains WORK_STRUCT_*
and WORK_OFFQ_* flags.  As WORK_OFFQ_* flags are used only while a
work item is off-queue, it can occupy bits of work->data which aren't
used while off-queue.  WORK_OFFQ_* currently only use bits used by
on-queue CWQ pointer.  As color bits aren't used while off-queue,
there's no reason to not use them.

Lower WORK_OFFQ_FLAG_BASE from WORK_STRUCT_FLAG_BITS to
WORK_STRUCT_COLOR_SHIFT thus giving 4 more bits to off-queue flag
space which is also used to record worker_pool ID while off-queue.

This doesn't introduce any visible behavior difference.

tj: Rewrote the description.
Signed-off-by: NLai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

include/linux/workqueue.h

@@ -68,7 +68,7 @@ enum {
 				  WORK_STRUCT_COLOR_BITS,
 
 	/* data contains off-queue information when !WORK_STRUCT_PWQ */
-	WORK_OFFQ_FLAG_BASE	= WORK_STRUCT_FLAG_BITS,
+	WORK_OFFQ_FLAG_BASE	= WORK_STRUCT_COLOR_SHIFT,
 
 	WORK_OFFQ_CANCELING	= (1 << WORK_OFFQ_FLAG_BASE),