[stable6] Fix trashbin warnings in logs

This fixes two things:
1) etag warnings in logs: https://github.com/owncloud/core/issues/6400
2) Backports https://github.com/owncloud/core/pull/7872 to avoid yet another warning about `disk_free_space`

@enoch85 can you help testing both cases ? :smile: 
See the referenced links for reproduction steps.

@karlitschek fixes for stable6 only, master is ok as the code is very different there.