kernel - Fix mbuf cluster statistics, fix type change bug

kernel - Fix mbuf cluster statistics, fix type change bug

* The mbuf cluster statistics were not properly handling a sharecount race case,
  causing the cluster count to continuously increase under heavy loads.

* atomic_set_short() was being improperly used to set m->m_type, causing the type field
  to collect a logical OR of changeouts.  Just set it normally.

* We don't need to use atomic ops for per-cpu stats updates.

Reported-by: Peter Avalos <peter@theshell.com>, "Samuel J. Greear" <sjg@evilcode.net>