rcu: Set default to RCU_CPU_STALL_INFO=y

The RCU_CPU_STALL_INFO code has been in for quite some time, and has
proven reliable.  This commit therefore enables it by default.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

lib/Kconfig.debug

@@ -1257,7 +1257,7 @@ config RCU_CPU_STALL_TIMEOUT
 config RCU_CPU_STALL_INFO
 	bool "Print additional diagnostics on RCU CPU stall"
 	depends on (TREE_RCU || PREEMPT_RCU) && DEBUG_KERNEL
-	default n
+	default y
 	help
 	  For each stalled CPU that is aware of the current RCU grace
 	  period, print out additional per-CPU diagnostic information