rcu: Create per-cpu rcuc kthreads only when rcutree.use_softirq=0

The per-CPU "rcuc" kthreads are used only by kernels booted with
rcutree.use_softirq=0, but they are nevertheless unconditionally created
by kernels built with CONFIG_RCU_BOOST=y.  This results in "rcuc"
kthreads being created that are never actually used.  This commit
therefore refrains from creating these kthreads unless the kernel
is actually booted with rcutree.use_softirq=0.

Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Zqiang <qiang1.zhang@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree.c

@@ -2894,7 +2894,7 @@ static int __init rcu_spawn_core_kthreads(void)
 
 	for_each_possible_cpu(cpu)
 		per_cpu(rcu_data.rcu_cpu_has_work, cpu) = 0;
-	if (!IS_ENABLED(CONFIG_RCU_BOOST) && use_softirq)
+	if (use_softirq)
 		return 0;
 	WARN_ONCE(smpboot_register_percpu_thread(&rcu_cpu_thread_spec),
 		  "%s: Could not start rcuc kthread, OOM is now expected behavior\n", __func__);