LU-14361 statahead: increase the initial statahead count

LU-14361 statahead: increase the initial statahead count

In this patch, we increase the initial stat-ahead count from the
default 8 to 64 during the fname statahead pattern test sanity/
test_123i. The origial starting statahead count is too small, may
result in that the statahead thread quits wrongly. This will fail
sanity/test_123i fairly often.

We also imporve aheadmany and use it to generate the fname stat()
workload to verify that fname statahead pattern works correctly.

Test-Parameters: mdtcount=4 mdscount=2 testlist=sanity env=ONLY=123i,ONLY_REPEAT=100
Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I7d13120a9480ea5b2e53963789074429c414ff90
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/53634
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>