rcu: Set rcutree.kthread_prio sysfs access to read-only

The rcutree.kthread_prio kernel-boot parameter is used to set the
priority for boost (rcub), per-CPU (rcuc), and grace-period (rcu_preempt
or rcu_sched) kthreads.  It is also used by rcutorture to check whether
it is possible to meaningfully test RCU priority boosting.  However,
all of these cases will either ignore or be confused by any post-boot
changes to rcutree.kthread_prio.

Note that the user really can change the priorities of all of these
kthreads using chrt, given sufficient privileges.  Therefore, the
read-write nature of sysfs access to rcutree.kthread_prio is thus at
best an attractive nuisance.

This commit therefore changes sysfs access to rcutree.kthread_prio to
be read-only.

Signed-off-by: Liu Song <liu.song11@zte.com.cn>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>