LU-2915 lfsck: NO scrub for agent inode and remote parent

Under DNE case, for the object on remote MDT, there will be an agent
inode on the local MDT where the name entry resides. We do NOT build
OI mapping for such agent inode, so OI scrub should skip it.The same
for the "/REMOTE_PARENT_DIR".

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I164473bcba80043800d491639c027e7c13f30bd9
Reviewed-on: http://review.whamcloud.com/6332
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: wangdi <di.wang@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>