Revert "sched: Remove synchronize rcu/sched calls from _cpu_down"

This reverts commit 36131fdc ("sched: Remove synchronize
rcu/sched calls from _cpu_down").

Removing the synchronization of rcu/sched calls from _cpu_down
introduces a race where tasks may get queued on an inactive CPU
and unthrottling cfs_rqs.

Change-Id: Ie29f8d185eb55979f9ca4e6e1b767caba6dd7f27
Signed-off-by: Mohammed Khajapasha <mkhaja@codeaurora.org>