info: Fix crashes when retrieving disk size

Info panel crashes has been fixed by !387 (merged) on master branch, however, gnome-3-30 is still affected as the same fix can't be used there, because its complexity and changed dependencies. The panel crashes because GUnixMountPoint is by mistake used as GUnixMountEntry. Let's find and use corresponing GUnixMountEntry to simply fix those crashes for gnome-3-30.

Closes: #411 (closed)