rcu: Don't spawn rcub kthreads on root rcu_node structure

Now that offlining CPUs no longer moves leaf rcu_node structures'
->blkd_tasks lists to the root, there is no way for the root rcu_node
structure's ->blkd_task list to be nonempty, unless the root node is also
the sole leaf node.  This commit therefore refrains from creating an rcub
kthread for the root rcu_node structure unless it is also the sole leaf.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree_plugin.h

@@ -1352,12 +1352,8 @@ static void __init rcu_spawn_boost_kthreads(void)
 	for_each_possible_cpu(cpu)
 		per_cpu(rcu_cpu_has_work, cpu) = 0;
 	BUG_ON(smpboot_register_percpu_thread(&rcu_cpu_thread_spec));
-	rnp = rcu_get_root(rcu_state_p);
-	(void)rcu_spawn_one_boost_kthread(rcu_state_p, rnp);
-	if (NUM_RCU_NODES > 1) {
 	rcu_for_each_leaf_node(rcu_state_p, rnp)
 		(void)rcu_spawn_one_boost_kthread(rcu_state_p, rnp);
-	}
 }
 
 static void rcu_prepare_kthreads(int cpu)