This is a child issue of #11529. I'm opening it because this will be very easy to achieve - the code that's behind the collection-level quotas is already implemented to cover all the dvobjectcontainers, i.e. both Collections and Datasets.
Producing this patch will allow us to at least quickly slap quotas on all the datasets sitting directly in the top-level collection at HDV. This will not address the issue of the same user creating multiple datasets in root - which needs to be fully addressed in the parent issue, but that will take more of an appreciable design and dev. effort.