LU-2253 tests: fix typo in test_2 of s-q

In test_2 of s-q, the awk column number should be changed from
5 to 4 when replacing the 'lfs df' with 'lfs_df'.

Test-Parameters: envdefinitions=SLOW=yes testlist=sanity-quota

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: Ia37df21915dd631c9ee7d8c46249291dc1a6f6b5
Reviewed-on: http://review.whamcloud.com/7341
Tested-by: Hudson
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>