turn bench_fastpath_throughput into actual throughput benchmark

Previously we bumped size by 16 between iterations, but for many size
classess that gave is subsequent iteration into same size
class. Multiplying by prime number randomizes sizes more so speeds up
this benchmark on at least modern x86.

benchmark/malloc_bench.cc

@@ -45,7 +45,7 @@ static void bench_fastpath_throughput(long iterations,
     free(p);
     // this makes next iteration use different free list. So
     // subsequent iterations may actually overlap in time.
-    sz = (sz & 511) + 16;
+    sz = ((sz * 8191) & 511) + 16;
   }
 }