LU-2739 mdt: Deny non-DNE client to access remote directory

LU-2739 mdt: Deny non-DNE client to access remote directory

Return EIO to old clients, if they try to access the remote
directory.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: I1cc4e8fa6e0f3a4b31b5610642e204bd100c1743
Reviewed-on: http://review.whamcloud.com/5425
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>