LU-14139 statahead: add total hit/miss count stats

LU-14139 statahead: add total hit/miss count stats

In this patch, it adds total hit/miss count stats for statahead.
These statistics are updated when the statahead thread terminated.

This patch also adds support to clear all statahead stats:
	$LCTL set_param llite.*.statahead_stats=0

Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I8b11d26385234305631c232a15711224dcfb0668
Reviewed-on: https://review.whamcloud.com/46309
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>