workqueue: reimplement work flushing using linked works

A work is linked to the next one by having WORK_STRUCT_LINKED bit set
and these links can be chained.  When a linked work is dispatched to a
worker, all linked works are dispatched to the worker's newly added
->scheduled queue and processed back-to-back.

Currently, as there's only single worker per cwq, having linked works
doesn't make any visible behavior difference.  This change is to
prepare for multiple shared workers per cpu.

Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/workqueue.h

@@ -24,8 +24,9 @@ typedef void (*work_func_t)(struct work_struct *work);
 
 enum {
 	WORK_STRUCT_PENDING_BIT	= 0,	/* work item is pending execution */
+	WORK_STRUCT_LINKED_BIT	= 1,	/* next work is linked to this one */
 #ifdef CONFIG_DEBUG_OBJECTS_WORK
-	WORK_STRUCT_STATIC_BIT	= 1,	/* static initializer (debugobjects) */
+	WORK_STRUCT_STATIC_BIT	= 2,	/* static initializer (debugobjects) */
 	WORK_STRUCT_COLOR_SHIFT	= 3,	/* color for workqueue flushing */
 #else
 	WORK_STRUCT_COLOR_SHIFT	= 2,	/* color for workqueue flushing */
@@ -34,6 +35,7 @@ enum {
 	WORK_STRUCT_COLOR_BITS	= 4,
 
 	WORK_STRUCT_PENDING	= 1 << WORK_STRUCT_PENDING_BIT,
+	WORK_STRUCT_LINKED	= 1 << WORK_STRUCT_LINKED_BIT,
 #ifdef CONFIG_DEBUG_OBJECTS_WORK
 	WORK_STRUCT_STATIC	= 1 << WORK_STRUCT_STATIC_BIT,
 #else