rcu: Restrict RCU_STRICT_GRACE_PERIOD to at most four CPUs

Kernels built with CONFIG_RCU_STRICT_GRACE_PERIOD=y can experience
significant lock contention due to RCU's resulting focus on ending grace
periods as soon as possible.  This is OK, but only if there are not very
many CPUs.  This commit therefore puts this Kconfig option off-limits
to systems with more than four CPUs.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/Kconfig.debug

@@ -116,7 +116,7 @@ config RCU_EQS_DEBUG
 
 config RCU_STRICT_GRACE_PERIOD
 	bool "Provide debug RCU implementation with short grace periods"
-	depends on DEBUG_KERNEL && RCU_EXPERT
+	depends on DEBUG_KERNEL && RCU_EXPERT && NR_CPUS <= 4
 	default n
 	select PREEMPT_COUNT if PREEMPT=n
 	help