backup: Ignore xattr.list permission error for parent directories

On FreeBSD, limited users may not be able to even list xattrs for the
parent directories above the snapshot source paths. As this can cause
the backup to fail, just ignore those errors.

internal/restic/node_openbsd.go

@@ -23,6 +23,10 @@ func Listxattr(path string) ([]string, error) {
 	return nil, nil
 }
 
+func IsListxattrPermissionError(_ error) bool {
+	return false
+}
+
 // Setxattr is a no-op on openbsd.
 func Setxattr(path, name string, data []byte) error {
 	return nil