rcu: Use kthread preferred affinity for RCU exp kworkers

Now that kthreads have an infrastructure to handle preferred affinity
against CPU hotplug and housekeeping cpumask, convert RCU exp workers 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>