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.

This patch is back-ported from the following one:
Lustre-commit: a1cf492e3632e2930b9593817d9397ba1b378fda
Lustre-change: http://review.whamcloud.com/9339

Signed-off-by: Jian Yu <jian.yu@intel.com>
Change-Id: I5c40e704266672af270e8306dff1fb867af1b8b5
Reviewed-on: http://review.whamcloud.com/9946
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>