LU-2388 statahead: don't statahead if it's stopped

LU-2388 statahead: don't statahead if it's stopped

Statahead may be stopped but lli->lli_sai is not NULL(inflight
statahead RPCs), so check lli->lli_opendir_key before accessing
statahead data.

Signed-off-by: Lai Siyao <laisiyao@whamcloud.com>
Change-Id: I4bcd4da0dc2913697473bf85ff6e502df43ce5a2
Reviewed-on: http://review.whamcloud.com/4718
Tested-by: Hudson
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Peng Tao <bergwolf@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>