support Fallocate on linux

weed/topology/allocate_volume.go

@@ -20,6 +20,7 @@ func AllocateVolume(dn *DataNode, vid storage.VolumeId, option *VolumeGrowOption
 	values.Add("collection", option.Collection)
 	values.Add("replication", option.ReplicaPlacement.String())
 	values.Add("ttl", option.Ttl.String())
+	values.Add("preallocate", fmt.Sprintf("%d", option.Prealloacte))
 	jsonBlob, err := util.Post("http://"+dn.Url()+"/admin/assign_volume", values)
 	if err != nil {
 		return err

weed/topology/volume_growth.go

@@ -21,6 +21,7 @@ type VolumeGrowOption struct {
 	Collection       string
 	ReplicaPlacement *storage.ReplicaPlacement
 	Ttl              *storage.TTL
+	Prealloacte      int64
 	DataCenter       string
 	Rack             string
 	DataNode         string