LU-18448 llite: read dir on open

LU-18448 llite: read dir on open

Let's read some pages at directory start,
a clients needs it probably.

walk over ~100k directories with 150 files on last leaf.

readdir on open enabled.

    real    0m39.977s
    user    0m0.121s
    sys     0m7.161s

readdir on open disabled

    real    1m18.106s
    user    0m0.151s
    sys     0m15.666s

HPE-bug-id: LUS-7695
Signed-off-by: Alexey Lyashkov <alexey.lyashkov@hpe.com>
Change-Id: Iaa674ce0d2e5723b380d7ca09407b27a90bc37f5
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57069
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andrew Perepechko <andrew.perepechko@hpe.com>
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>