Fixing free drive space issue in webadmin, close #11570

Fixing https://trac.openmicroscopy.org.uk/ome/ticket/11570

Testing:
Log in as root to webadmin and in Statistics check Free space just under the pie-chart
Log in again as a user, go to User settings and check if in Statistics is exactly the same value

Previously, mismatch example: 
 - User: Free space: 225770 GB
 - Admin: Free space: 220.48 GB