Bugfix: Support exporting a `restic mount` of a Windows system via Samba

A repository mounted using `restic mount` on a POSIX system could not use
Samba to export files from restic backups of Windows systems. Backups of
other systems were not affected. This has been fixed.

https://github.com/restic/restic/issues/2034
https://github.com/restic/restic/issues/4382
https://github.com/restic/restic/pull/21784
