gc: fix integer overflow when computing how much to GC

gc: fix integer overflow when computing how much to GC

On 32-bit systems the insufficient GC could commonly happen:
https://lists.nic.cz/pipermail/knot-resolver-users/2020/000265.html

The meaning of -f parameter got slightly changed, so that the buggy
computation could be greatly simplified.  GC seems to make sense when
most of cache space is used, in which case the difference is small.