LU-14927 osd: share brw_stats code between OSD back ends.

LU-14927 osd: share brw_stats code between OSD back ends.

Both the ldiskfs and ZFS OSD backend handle brw_stats. With the
stricter GPL requirement ZFS can no longer carry the brw_stats
code. So move the common code to lprocfs_status_server.c as
well as move brw_stats to debugfs as well.

Change-Id: I294e5df3557552266dd3a02d3bc9844c42c01f60
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/44690
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Aurelien Degremont <degremoa@amazon.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>