archiver: use ExtendedStat from FS interface

With this change, NodeFromFileInfo is the last function that bypasses
the FS interface in the archiver.

internal/fs/fs_reader.go

@@ -126,6 +126,12 @@ func (fs *Reader) DeviceID(_ os.FileInfo) (deviceID uint64, err error) {
 	return 0, errors.New("Device IDs are not supported")
 }
 
+func (fs *Reader) ExtendedStat(fi os.FileInfo) ExtendedFileInfo {
+	return ExtendedFileInfo{
+		FileInfo: fi,
+	}
+}
+
 // Join joins any number of path elements into a single path, adding a
 // Separator if necessary. Join calls Clean on the result; in particular, all
 // empty strings are ignored. On Windows, the result is a UNC path if and only