EX-8130 lipe: Add key func for work with tree

EX-8130 lipe: Add key func for work with tree

This patch adds two key functions to collect directory size
statistics, which contain the basic logic for adding
directories to memory and incrementing size counters.

Test-Parameters: trivial
Signed-off-by: Vitaliy Kuznetsov <vkuznetsov@ddn.com>
Change-Id: I06e03a6be1052b7178274835169cc41d044ca1ab
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/53963
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>