Bugfix: Fix erroneous warnings about unsupported fsync

Due to a regression in rest-server 0.12.0, it continuously printed
`WARNING: fsync is not supported by the data storage. This can lead to data loss,
if the system crashes or the storage is unexpectedly disconnected.` for systems
that support fsync. We have fixed the warning.

https://github.com/restic/rest-server/issues/230
https://github.com/restic/rest-server/pull/231