rcu: Use kthread preferred affinity for RCU boost

Now that kthreads have an infrastructure to handle preferred affinity
against CPU hotplug and housekeeping cpumask, convert RCU boost to use
it instead of handling all the constraints by itself.

Acked-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>