cascache: limit 'infinite' cache to volume size

Make sure we don't exceed the size of the volume when calculating an 'unlimited' quota.

Note that at present, cache_size is an estimate that will be the same as the cache_size or larger. In my own case, I have a volume dedicated to the cache, so it was easy for the cache_quota to exceed this. This prevented me from running BuildStream, although I had plenty of free space.

Fixes #906 (closed)