LU-2694 test: improve lfsck.sh

LU-2694 test: improve lfsck.sh

The test directory of lfsck.sh contains some files referencing
same object, which could cause error when removing the directory
on test cleanup.

Test-Parameters: testlist=lfsck
Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I334ff2b7b5f77498eed940f009e4bc18728bb5da
Reviewed-on: http://review.whamcloud.com/5606
Tested-by: Hudson
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>