Now also preventing to override "files" dir size with -1

Fixes #6526

Please review @icewind1991 @karlitschek @DeepDiver1975 

The past fix for insufficient storage was put in HomeCache and prevented the size of "/" to be overwritten by -1.
Then, a later fix changed the logic to use the size of "/files" instead of "/" for the free size calculation (when quota is enabled), but the "overwrite with -1 prevention" code hadn't been adapted, so there is still a risk that the size of "/files" becomes -1.
This fix should prevent that and hopefully remove those annoying messages.

Note that I'm unable to test whether it fixes the issue, but people who had the issue are helping to verify it (see #6526)

Will be backported to stable6 after merging.