LU-3367 test: some fixes for lfsck test

LU-3367 test: some fixes for lfsck test

This patch includes the following fixes:
- the patch of LU-2780 (f9659fd87771122d01a4fafb09ba59c4cf0091ab),
changed ./lustre/fid to be an real inode, which breaks
is_empty_fs() of t-f, so update is_empty_fs() accordingly and add
version check code to check this change.
- some code cleanup
Part of this patch comes from
LU-3180 http://review.whamcloud.com/#change,6123 .

Test-Parameters: testlist=lfsck
Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Signed-off-by: Liu Ying <emoly.liu@intel.com>
Change-Id: Ic9f517ca4adcf01e191a62c6a16b097ebcb4c9a5
Reviewed-on: http://review.whamcloud.com/6494
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>