rcu: Hide RCU_NOCB_CPU behind RCU_EXPERT

This commit prevents Kconfig from asking the user about RCU_NOCB_CPU
unless the user really wants to be asked.
Reported-by: NIngo Molnar <mingo@kernel.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Thomas Gleixner <tglx@linutronix.de>

init/Kconfig

@@ -698,6 +698,7 @@ config RCU_BOOST_DELAY
 config RCU_NOCB_CPU
 	bool "Offload RCU callback processing from boot-selected CPUs"
 	depends on TREE_RCU || PREEMPT_RCU
+	depends on RCU_EXPERT || NO_HZ_FULL
 	default n
 	help
 	  Use this option to reduce OS jitter for aggressive HPC or