LU-3716 obdecho: create a separate root object for echo access

Currently, while echo client and normal client are attached at the
same time, both md echo objects and normal objects are created
and looked up under the same root object (ROOT), which will cause
ASSERTION( lu_device_is_mdt(o->lo_dev) ) failure.

This patch fixes the issue by creating a separate root object
(ROOT_ECHO) for echo access. The md echo objects created under
this root object can only be accessed by echo client. Normal client
will never see these echo objects.

Test-Parameters: alwaysuploadlogs envdefinitions=SLOW=yes \
mdtcount=1 testlist=mds-survey

Signed-off-by: Jian Yu <jian.yu@intel.com>
Change-Id: I8d8a9bd2c467bb40a7993d492aa3d4ba6676ac8f
Reviewed-on: http://review.whamcloud.com/10130
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: wangdi <di.wang@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>