backup: convert reject funcs to use FS interface

Depending on parameters the paths in a snapshot do not directly
correspond to real paths on the filesystem. Therefore, reject funcs must
use the FS interface to work correctly.

internal/fs/fs_reader.go

@@ -122,6 +122,10 @@ func (fs *Reader) Lstat(name string) (os.FileInfo, error) {
 	return nil, pathError("lstat", name, os.ErrNotExist)
 }
 
+func (fs *Reader) DeviceID(_ os.FileInfo) (deviceID uint64, err error) {
+	return 0, errors.New("Device IDs are not supported")
+}
+
 // 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