LU-9930 llite: only clear lli_sai if the setter

LU-9930 llite: only clear lli_sai if the setter

Previous to this patch, start_statahead_thread() was
unconditionnally clearing lli->lli_sai upon error, leading
to crash upon racy scenario where it has just been set in/by
another thread context.
Now, only clear lli_sai if current thread has set it.

Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: I555febfad3494c9dd90eeb72d6dd9157428179ea
Reviewed-on: https://review.whamcloud.com/28794
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>