sched: Fix endless sync_sched/rcu() loop inside _cpu_down()

Commit:

sched: Remove get_online_cpus() usage

tries to do sync_sched/rcu() inside _cpu_down() but triggers:

INFO: task swapper/0:1 blocked for more than 120 seconds.
...
[<ffffffff811263dc>] synchronize_rcu+0x2c/0x30
[<ffffffff81d1bd82>] _cpu_down+0x2b2/0x340
...

It was caused by that in the rcu boost case we rely on smpboot thread to
finish the rcu callback, which has already been parked before sync in
here
and leads to the endless sync_sched/rcu().

This patch exchanges the sequence of smpboot_park_threads() and
sync_sched/rcu() to fix the bug.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Tested-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Michael Wang <wangyun@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/5282EDC0.6060003@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>

idea is taken here -
https://github.com/franciscofranco/flo/commit/dfba39d6808cac06e8697321f4ba9008da816de4