set the volume size limit
set the volume size limit

fixing https://github.com/chrislusf/seaweedfs/issues/447

chrislusf/seaweedfs,chrislusf/seaweedfs,k85169336/seaweedfs,hxiaodon/seaweedfs,chrislusf/seaweedfs,chrislusf/seaweedfs,k85169336/seaweedfs,hxiaodon/seaweedfs