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.

Lustre-change: https://review.whamcloud.com/29817


Lustre-commit: 8b1bd1b8

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Change-Id: Iaedddd3659cdffeab51800f45b02f0b39c4a1ec1
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Minh Diep <minh.diep@intel.com>
Reviewed-on: https://review.whamcloud.com/32366


Tested-by: Jenkins
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>