rcu: Eliminate ->onoff_mutex from rcu_node structure

Because that RCU grace-period initialization need no longer exclude
CPU-hotplug operations, this commit eliminates the ->onoff_mutex and
its uses.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --cc kernel/rcu/tree.c
Simple merge

diff --cc kernel/rcu/tree.h
Simple merge