LU-2682 fid: cleanup direct _id and _seq access

LU-2682 fid: cleanup direct _id and _seq access

Access/copying the individual _id and _seq fields should be
replaced with the _oi field, and compatibility macros for
accessing the individual fields should be removed.

Signed-off-by: Wang Di <di.wang@intel.com>
Change-Id: Iedce5cfb308c1b6de6aa24872b8c71e7b616b67c
Reviewed-on: http://review.whamcloud.com/5601
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>