LU-16762 statahead: wait until statahead thread quit

LU-16762 statahead: wait until statahead thread quit

It must wait until statahead thread quit. After that, we can get
accurate hit/miss stats for stat() workload such as "ls -l".

Test-Parameters: trivial
Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I902b299e039de6c584b386856fb3f7a8989eb73b
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52302
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>