Bugfix: `backup` works if xattrs above the backup target cannot be read

When backup targets are specified using absolute paths, then `backup` also
includes information about the parent folders of the backup targets 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 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
