LU-5855 lfsck: misc fixes for zfs-based backend

LU-5855 lfsck: misc fixes for zfs-based backend

It contains several fixes to make the LFSCK to work under DNE mode
for zfs-based backend.

Test-Parameters: mdsfilesystemtype=zfs mdtfilesystemtype=zfs ostfilesystemtype=zfs mdscount=2 mdtcount=2 testlist=sanity-lfsck
Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I8e8758336d4ce67667f7e3586475ddd72db2d419
Reviewed-on: http://review.whamcloud.com/12552
Tested-by: Jenkins
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: wangdi <di.wang@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>