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

This reverts commit c9e8afb8.

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: I3a24314a403fc7352f506cda0016963621c79f6a
Signed-off-by: Kyle Yan <kyan@codeaurora.org>