Bugfix: Skip extended attribute processing on unsupported Windows volumes

With restic 0.17.0, backups of certain Windows paths, such as network drives,
failed due to errors while fetching extended attributes.

Restic now skips extended attribute processing for volumes where they are not
supported.

https://github.com/restic/restic/pull/4980
https://github.com/restic/restic/pull/4998
https://github.com/restic/restic/issues/4955
https://github.com/restic/restic/issues/4950
