Fix stats.cactive accounting regression.

Fix stats.cactive accounting to always increase/decrease by multiples of
the chunk size, even for huge size classes that are not multiples of the
chunk size, e.g. {2.5, 3, 3.5, 5, 7} MiB with 2 MiB chunk size.  This
regression was introduced by 155bfa7d
(Normalize size classes.) and first released in 4.0.0.

This resolves #336.