LU-2396 ofd: Remove unmaintained brw_stats

LU-2396 ofd: Remove unmaintained brw_stats

Remove unused nid_brw_stats member from struct nid_stat and the files
/proc/fs/lustre/obdfilter/*/{brw_stats,exports/*/brw_stats}.  If the
corresponding osd has a brw_stats proc entry then add a symlink from
the ofd proc dir.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I79a10816c612f67b80172d679b875ca94157b5f4
Reviewed-on: http://review.whamcloud.com/4689
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Hudson
Reviewed-by: Keith Mannthey <keith.mannthey@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>