rcu: Use smp_hotplug_thread facility for RCUs per-CPU kthread

Bring RCU into the new-age CPU-hotplug fold by modifying RCU's per-CPU
kthread code to use the new smp_hotplug_thread facility.

[ tglx: Adapted it to use callbacks and to the simplified rcu yield ]

idea is taken here -
https://github.com/franciscofranco/flo/commit/4c6b41ce533591106a1407dbf6d27d26146821e2