LU-3415 quota: not enforce quota when accounting disabled

When quota accounting isn't enabled, don't try to enforce quota.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I9a74b7bfe6b2981adf501042efb6867a77ee4044
Reviewed-on: http://review.whamcloud.com/6492
Tested-by: Hudson
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>