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_local.go

@@ -52,6 +52,11 @@ func (fs Local) DeviceID(fi os.FileInfo) (id uint64, err error) {
 	return deviceID(fi)
 }
 
+// ExtendedStat converts the give FileInfo into ExtendedFileInfo.
+func (fs Local) ExtendedStat(fi os.FileInfo) ExtendedFileInfo {
+	return ExtendedStat(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