EX-7729 osc: Add counters for compressed data

EX-7729 osc: Add counters for compressed data

This patch is the first of two patches that add counters
to track client/server-side data compression statistics.
This patch add new compr_stats file in osc.*.compr_stats.

From added counters:
1. Size of compressed/uncompressed chunks written/read by
   client to compressed files, in chunk/bytes;
2. Compressed page counter;

Test-Parameters: trivial testlist=sanity-compr
Signed-off-by: Vitaliy Kuznetsov <vkuznetsov@ddn.com>
Change-Id: I091153480e53309c641d39f271bef536296dc09e
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/53737
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Artem Blagodarenko <ablagodarenko@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>