LU-6999 osp: disable xattr cache for mdt obj

LU-6999 osp: disable xattr cache for mdt obj

Because ldlm lock is missing for xattr cache on OSP obj,
let's Disable xattr cache for remote MDT object.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: I4a7fa5f13791d531309f380f9e9303d04816c6a9
Reviewed-on: http://review.whamcloud.com/15974
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>