fix unclaimed spaces calculation when volumePreallocate is enabled (#6063)

the calculation of `unclaimedSpaces` only needs to subtract `unusedSpace` when `preallocate` is not enabled.

Signed-off-by: LHHDZ <shichanglin5@qq.com>

weed/server/master_grpc_server.go

@@ -151,6 +151,7 @@ func (ms *MasterServer) SendHeartbeat(stream master_pb.Seaweed_SendHeartbeatServ
 
 			if err := stream.Send(&master_pb.HeartbeatResponse{
 				VolumeSizeLimit: uint64(ms.option.VolumeSizeLimitMB) * 1024 * 1024,
+				Preallocate:     ms.preallocateSize > 0,
 			}); err != nil {
 				glog.Warningf("SendHeartbeat.Send volume size to %s:%d %v", dn.Ip, dn.Port, err)
 				return err