Fix/simplify extent_recycle() allocation size computations.

Do not call s2u() during alloc_size computation, since any necessary
ceiling increase is taken care of later by extent_first_best_fit() -->
extent_size_quantize_ceil(), and the s2u() call may erroneously cause a
higher quantization result.

Remove an overly strict overflow check that was added in
4a785213 (Fix extent_recycle()'s
cache-oblivious padding support.).