LU-7156 mdd: add changelog_size to procfs

LU-7156 mdd: add changelog_size to procfs

It is hard to get size of changelogs. This patch adds new procfs file
changelog_size. It shows the current size of all changelog files.

Signed-off-by: Alexander Boyko <alexander.boyko@seagate.com>
Seagate-bug-id: MRP-2383
Change-Id: Ia2905bbf965256b18708fb79c9e5e07b4ffd2b10
Reviewed-on: http://review.whamcloud.com/16416
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>