LU-2694 test: fix is_empty_fs in t-f

LU-2694 test: fix is_empty_fs in t-f

The original is_empty_fs is incorrect, which can cause unexpected
error when running lfsck.sh, because lfsck sets the max error
level based on whether filesystem is empty.

Test-Parameters: testlist=lfsck
Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I2269cc41744e3c9fe228898323a1508a03616efe
Reviewed-on: http://review.whamcloud.com/5576
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Tested-by: Hudson
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>