LU-3180 tests: misc fix in lfsck.sh

- Make sure data flush back before run lfsck;
- Go back to old lfsck behaviour: supress the error of the second run
  of lfsck;
- Fix typo in is_empty_fs();

Test-Parameters: envdefinitions=SLOW=yes testlist=lfsck
Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I6216b5325ff7d9dc420d57a11f6d09f345bff96c
Reviewed-on: http://review.whamcloud.com/6123
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>