LU-10165 llite: disable statahead if starting statahead fail

LU-10165 llite: disable statahead if starting statahead fail

Once starting statahead thread fails, it should disable statahead.
Current code only does this when "sai != NULL", instead it should
check whether current process is opening this dir, so for cases
like current file is not the first dirent, or sai allocation fail,
it won't retry statahead.

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Change-Id: Iaedddd3659cdffeab51800f45b02f0b39c4a1ec1
Reviewed-on: https://review.whamcloud.com/29817
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>