LU-4598 quota: fix s-q test_30

LU-4598 quota: fix s-q test_30

After LU-4139 landed, the block grace time isn't accurate as before,
the s-q test_30 should now write more bytes to make sure the spare
quota allocated on slave be used up.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: Iee14721051b9e41074a13f11afb11a7b286352c2
Reviewed-on: http://review.whamcloud.com/9177
Tested-by: Jenkins
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>