LU-14361 statahead: add support for mdtest shared dir workload

LU-14361 statahead: add support for mdtest shared dir workload

This patch adds statahead support for shared dir stat() workload
with fname patteren like mdtest shared dir stat() access.

The performance imporvements are shown as follows:
IO500 (KIOPS)		w/o patch	w/ path
mdtest-easy-stat	740.01		1276.31
mdtest-hard-stat	514.36		1105.33

Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I43983e91eb864bd317cfb883e35e2f4c1a8f788c
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/48954
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>