rcu: Wire up RCU_BOOST_PRIO for rcutree

RCU boost threads start life at RCU_BOOST_PRIO, while others remain
at RCU_KTHREAD_PRIO.  While here, change thread names to match other
kthreads, and adjust rcu_yield() to not override the priority set by
the user.  This last change sets the stage for runtime changes to
priority in the -rt tree.

Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --cc kernel/rcutree.c
Simple merge

diff --cc kernel/rcutree_plugin.h
Simple merge