rcu: allow RCU CPU stall warning messages to be controlled in /sys

Set the permissions of the rcu_cpu_stall_suppress to 644 to enable RCU
CPU stall warnings to be enabled and disabled at runtime via sysfs.
Suggested-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcutree.c

@@ -145,7 +145,7 @@ module_param(qlowmark, int, 0);
 
 #ifdef CONFIG_RCU_CPU_STALL_DETECTOR
 int rcu_cpu_stall_suppress __read_mostly;
-module_param(rcu_cpu_stall_suppress, int, 0);
+module_param(rcu_cpu_stall_suppress, int, 0644);
 #endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
 
 static void force_quiescent_state(struct rcu_state *rsp, int relaxed);