EX-8130 lipe: Add entry point for dirs stats

EX-8130 lipe: Add entry point for dirs stats

This pr adds a function that is an entry point for
collecting statistics about directory sizes.

Test-Parameters: trivial
Signed-off-by: Vitaliy Kuznetsov <vkuznetsov@ddn.com>
Change-Id: Ide0c6006e287f69a1de99a5578ceab0070ea383e
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/53982
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alexandre Ioffe <aioffe@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>