Fix arena data structure size calculation.

Fix paren placement so that QUANTUM_CEILING() applies to the correct
portion of the expression that computes how much memory to base_alloc().
In practice this bug had no impact.  This was caused by
5d8db15d (Simplify run quantization.),
which in turn fixed an over-allocation regression caused by
3c4d92e8 (Add per size class huge
allocation statistics.).