rcu: Make nocb_nobypass_lim_per_jiffy static

RCU triggerse the following sparse warning:

kernel/rcu/tree_plugin.h:1497:5: warning: symbol
'nocb_nobypass_lim_per_jiffy' was not declared. Should it be static?

This commit therefore makes this variable static.
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Reported-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree_plugin.h

@@ -1556,7 +1556,7 @@ early_param("rcu_nocb_poll", parse_rcu_nocb_poll);
  * After all, the main point of bypassing is to avoid lock contention
  * on ->nocb_lock, which only can happen at high call_rcu() rates.
  */
-int nocb_nobypass_lim_per_jiffy = 16 * 1000 / HZ;
+static int nocb_nobypass_lim_per_jiffy = 16 * 1000 / HZ;
 module_param(nocb_nobypass_lim_per_jiffy, int, 0);
 
 /*