info: Fix crashes when retrieving disk size

Info panel crashes has been fixed by !387 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
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