rcu: avoid hammering sched with yet another bound RT kthread

The scheduler does not appear to take kindly to having multiple
real-time threads bound to a CPU that is going offline.  So this
commit is a temporary hack-around to avoid that happening.

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