LU-4139 quota: improve write performance when over softlimit

To improve the write performance when over softlimit, we choose
to lose some grace time accuracy: shrinking the qunit size to
(4 * least_qunit_size) instead of least_qunit_size.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I20e341d53979ac42227197ec6fa04dd38974073f
Reviewed-on: http://review.whamcloud.com/8078
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>