kernel - Refactor struct vmstats and vm_zone

kernel - Refactor struct vmstats and vm_zone

* These changes significantly improve the simultaneous non-conflicting
  VM fault rate.  On our 4-socket opteron (48 cores, which makes a great
  test case because its cache mastership stalls are so expensive), the
  maximum concurrent VM fault rate increased from ~2.4M/sec to ~3.5M/sec,
  and suffers no degredation after topping out.

* Refactor the fields in struct vmstats to separate out mostly
  read-only variables from nominally modified variables, reducing
  cache mastership stalls.

* Remove vm_shared_hit, vm_shared_count, and vm_shared_miss sysctl
  statistics, removing related cache mastership stalls from the
  critical path.

* Move the spinlock in vpgqueues to the base of the structure.

* Increase the vmstats slop (how large a negative value can accumulate
  in pcpu stats before rolling it up).

* Fix cache mastership stalls in the zalloc() and zfree() paths by
  consolidating pcpus elements into its own cache-aligned structure
  and giving each pcpu its on znalloc counter.