rcu: Don't initiate RCU priority boosting on root rcu_node

Because there is no longer any preempted tasks on the root rcu_node, and
because there is no longer ever an rcub kthread for the root rcu_node,
this commit drops the code in force_qs_rnp() that attempts to awaken
the non-existent root rcub kthread.  This is strictly a performance
enhancement, removing a root rcu_node ->lock acquisition and release
along with some tests in rcu_initiate_boost(), ending with the test that
notes that there is no rcub kthread.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.c

@@ -2513,12 +2513,6 @@ static void force_qs_rnp(struct rcu_state *rsp,
 		}
 		raw_spin_unlock_irqrestore(&rnp->lock, flags);
 	}
-	rnp = rcu_get_root(rsp);
-	if (rnp->qsmask == 0) {
-		raw_spin_lock_irqsave(&rnp->lock, flags);
-		smp_mb__after_unlock_lock();
-		rcu_initiate_boost(rnp, flags); /* releases rnp->lock. */
-	}
 }
 
 /*