LU-7994 statahead: add smp_mb() to serialize ops

LU-7994 statahead: add smp_mb() to serialize ops

In ll_deauthorize_statahead(), it set thread stop flag, and then
wake up thread, however wakeup is called inside spinlock in case
ll_statahead_info is released, then we need to call smp_mb() to
serialize setting and wakeup.

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Change-Id: I500bdf325aa0bd122b17bea448c3b9eec809a2a7
Reviewed-on: https://review.whamcloud.com/23040
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>