LU-4422 quota: fix s-q test_6

LU-4422 quota: fix s-q test_6

s-q test_6 should not fail by checking whether the file is
growing, because in most time, the test is waiting for quota
slave reconnecting to quota master.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I2332febf639bdbe033c588b97a2c7a1e1e2f51e2
Reviewed-on: http://review.whamcloud.com/9339
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>