LU-10190 osd-zfs: create agent object for remote object

LU-10190 osd-zfs: create agent object for remote object

In DNE environment, the object and its name entry may reside
on different MDTs. Under such case, we will create an agent
object on the MDT where the name entry resides. The agent
object is empty to indicates that the real object for this
name entry resides on another MDT. If without agent object,
related name entry will be skipped when perform MDT side file
level backup/restore via ZPL by userspace tool, such as 'tar'.

Test-Parameters: envdefinitions=ONLY=803 testlist=sanity mdtfilesystemtype=zfs ostfilesystemtype=zfs mdscount=2 mdtcount=4
Test-Parameters: envdefinitions=ONLY=34 testlist=sanity-lfsck mdtfilesystemtype=zfs ostfilesystemtype=zfs mdscount=2 mdtcount=4
Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I7fb8c54ca774f3877bcd326bc10d8e99083ac90a
Reviewed-on: https://review.whamcloud.com/28855
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>