Fix/simplify chunk_recycle() allocation size computations.

Remove outer CHUNK_CEILING(s2u(...)) from alloc_size computation, since
s2u() may overflow (and return 0), and CHUNK_CEILING() is only needed
around the alignment portion of the computation.

This fixes a regression caused by
5707d6f9 (Quantize szad trees by size
class.) and first released in 4.0.0.

This resolves #497.