fix compilation problem

weed/storage/types/offset_4bytes.go

@@ -12,7 +12,7 @@ type OffsetHigher struct {
 
 const (
 	OffsetSize            = 4
-	MaxPossibleVolumeSize = 4 * 1024 * 1024 * 1024 * 8 // 32GB
+	MaxPossibleVolumeSize uint64 = 4 * 1024 * 1024 * 1024 * 8 // 32GB
 )
 
 func OffsetToBytes(bytes []byte, offset Offset) {