workqueue: Make default affinity_scope dynamically updatable

While workqueue.default_affinity_scope is writable, it only affects
workqueues which are created afterwards and isn't very useful. Instead,
let's introduce explicit "default" scope and update the effective scope
dynamically when workqueue.default_affinity_scope is changed.

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

include/linux/workqueue.h

@@ -126,6 +126,7 @@ struct rcu_work {
 };
 
 enum wq_affn_scope {
+	WQ_AFFN_DFL,			/* use system default */
 	WQ_AFFN_CPU,			/* one pod per CPU */
 	WQ_AFFN_SMT,			/* one pod poer SMT */
 	WQ_AFFN_CACHE,			/* one pod per LLC */
@@ -133,8 +134,6 @@ enum wq_affn_scope {
 	WQ_AFFN_SYSTEM,			/* one pod across the whole system */
 
 	WQ_AFFN_NR_TYPES,
-
-	WQ_AFFN_DFL = WQ_AFFN_CACHE,
 };
 
 /**