LU-12615 mdt: check mdt_object

LU-12615 mdt: check mdt_object

In processing RPC of getattr, getxattr, swap_layouts and sync,
the mdt_object should be checked to verify there is a valid
RMF_MDT_BODY field and OBD_MD_FLID is set properly.

Change-Id: Ibb6aaa5ec5eb4b7284f4d5567a618a908d66920c
Signed-off-by: Hongchao Zhang <hongchao@whamcloud.com>
Reported-by: Alibaba Cloud <yunye.ry@alibaba-inc.com>
Reviewed-on: https://review.whamcloud.com/35764
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Emoly Liu <emoly@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>