rcu: fix a performance regression

Commit "rcu: Create RCU-specific workqueues with rescuers" switched RCU
to using local workqueses and removed power efficiency flag from them.

This caused a performance regression that can be observed in Geekbench 5
after enabling CONFIG_WQ_POWER_EFFICIENT_DEFAULT: score went down from
760/2500 to 620/2300 (single/multi core respectively).

Add WQ_POWER_EFFICIENT flag to avoid this regression.

kernel/rcu/tree.c

@@ -3569,9 +3569,9 @@ void __init rcu_init(void)
 	}
 
 	/* Create workqueue for expedited GPs and for Tree SRCU. */
-	rcu_gp_wq = alloc_workqueue("rcu_gp", WQ_MEM_RECLAIM, 0);
+	rcu_gp_wq = alloc_workqueue("rcu_gp", WQ_POWER_EFFICIENT | WQ_MEM_RECLAIM, 0);
 	WARN_ON(!rcu_gp_wq);
-	rcu_par_gp_wq = alloc_workqueue("rcu_par_gp", WQ_MEM_RECLAIM, 0);
+	rcu_par_gp_wq = alloc_workqueue("rcu_par_gp", WQ_POWER_EFFICIENT | WQ_MEM_RECLAIM, 0);
 	WARN_ON(!rcu_par_gp_wq);
 	srcu_init();
 }