Increase reported size of root dir

E.g. sshfs on MacOS stats the size of the root dir of the mount and rejects uploads larger than the available free space there. Thus, it would be great if mammutfs would report more than 63MiB free space:

Filesystem             Size   Used  Avail Capacity iused               ifree %iused  Mounted on
daten.stusta.mhn.de:  263Mi  200Mi   63Mi    77%     908                 100   90%   /none/of/your/business