kernel - Reduce the size of the callout wheel

kernel - Reduce the size of the callout wheel

* The callout wheel is per-cpu but ncallout is calculated based on memory.
  A system with many cpus tended to allocate an excessive amount of memory
  in aggregate for the callout wheels.

* Reduce the size of the per-cpu callout wheel by approximately a factor
  of (ncpus).  On a 16G machine with 8 cores, aggregate callout wheel
  allocations is reduced from 128MB to 16MB.