Bugfix: Handle unreadable xattrs in folders above `backup` source

When backup sources are specified using absolute paths, `backup` also includes
information about the parent folders of the backup sources in the snapshot.

If the extended attributes for some of these folders could not be read due to
missing permissions, this caused the backup to fail. This has now been fixed.

https://github.com/restic/restic/issues/3600
https://github.com/restic/restic/pull/4668
https://forum.restic.net/t/parent-directories-above-the-snapshot-source-path-fatal-error-permission-denied/7216
