rcu: Make RCU_BOOST default on CONFIG_PREEMPT_RT

On PREEMPT_RT kernels, RCU callbacks are deferred to the `rcuc' kthread.
This can stall RCU grace periods due to lengthy preemption not only of RCU
readers but also of 'rcuc' kthreads, either of which prevent grace periods
from completing, which can in turn result in OOM.  Because PREEMPT_RT
kernels have more kthreads that can block grace periods, it is more
important for such kernels to enable RCU_BOOST.

This commit therefore makes RCU_BOOST the default on PREEMPT_RT.
RCU_BOOST can still be manually disabled if need be.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/Kconfig

@@ -188,8 +188,8 @@ config RCU_FAST_NO_HZ
 
 config RCU_BOOST
 	bool "Enable RCU priority boosting"
-	depends on RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT
-	default n
+	depends on (RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT) || PREEMPT_RT
+	default y if PREEMPT_RT
 	help
 	  This option boosts the priority of preempted RCU readers that
 	  block the current preemptible RCU grace period for too long.