EX-8130 lipe: Add helper functions for stats

EX-8130 lipe: Add helper functions for stats

This patch adds several helper functions for working with
directory size statistics. Also add ls3_stats_rm_first_dir()
which remove the directory from the list to increase counters.

Test-Parameters: trivial
Signed-off-by: Vitaliy Kuznetsov <vkuznetsov@ddn.com>
Change-Id: I04807846b49d6fb0e476b8bf146ba337f80e3d5e
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/53962
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>