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
5d8db15db91c85d47b343cfc07fc6ea736f0de48 (Simplify run quantization.),
which in turn fixed an over-allocation regression caused by
3c4d92e82a31f652a7c77ca937a02d0185085b06 (Add per size class huge
allocation statistics.).
1 file changed