Add work around to the reboot issue of SMP, with
SMP, all the CPUs need to do _rcu_barrier, if we
enqueue an rcu callback, we need to make sure CPU
tick to stay alive until we take care of those by
completing the appropriate grace period.

This work around only work when the reboot command
issue, so it didn't impact normal kernel feature.

Signed-off-by: Anson Huang <b20788@freescale.com>