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>