EX-9280 lipe: extend periodic stats in lpurge

EX-9280 lipe: extend periodic stats in lpurge

In lpurge added periodic stats:
- Size and number of files which are not purged due to
   - stale
   - not mirrored
- Number of inodes total and used

These stats are refreshed with each purge cycle.  For example:

  testfs-OST0000: INFO: used_kb: 179564 (3%) total_kb: 5496292
                        used_inodes: 301 (0%) total_inodes: 375360
  testfs-OST0000: INFO: purged: 1 (20480KB 0%) failed_del: 0 (0KB 0%)
                        stale: 0 (0KB 0%) nomirror: 2 (178176KB 3%)

Test-Parameters: trivial testlist=hot-pools
Signed-off-by: Alexandre Ioffe <aioffe@ddn.com>
Change-Id: Ib404afe2b9d636bf1deaf8948411616971443932
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/55248
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>