LU-2646 osd: add special flag for agent inode.

LU-2646 osd: add special flag for agent inode.

1. Add special flag into the LMA of agent inode of remote entry,
so lfsck check can skip this inode.

2. Assign a local FID for Agent dir, so lfsck can skip this agent
dir as well.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: Ifc50fbc531a4306a76d5263dd6135c613d7a2a30
Reviewed-on: http://review.whamcloud.com/5625
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>