LU-14361 statahead: add tunable for fname pattern statahead

LU-14361 statahead: add tunable for fname pattern statahead

This patch adds a tunable parameter for fname pattern statahead.
Currenty fname pattern statahead is disabled.
It will be enabled by default once the patch series of fname
pattern statahead work is finished and stable.

Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I7a207e66df10b45bff1b3993e8724116489365b7
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/51572
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>