sched/core: Improve CPU hotplug latency for rcu_expedited=1 case

The sched_cpu_deactivate()->synchronize_rcu_mult() is not taking
expedited path even though rcu_expedited is enabled. So split
the synchronize_rcu_mult() call into synchronize_sched() and
synchronize_rcu() to improve the hotplug latency.

Change-Id: Ic21fbf790e7ed2f314b4058a489cde7813897c1f
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>