Make reference counting for mbuf clusters [only] work like in RELENG_4.
While I don't think this is the best solution, it certainly is the
fastest and in trying to find bottlenecks in network related code
I want this out of the way, so that I don't have to think about it.
What this means, for mbuf clusters anyway is:

• one less malloc() to do for every cluster allocation (replaced with a relatively quick calculation + assignment)
• no more free() in the cluster free case (replaced with empty space) :-)

This can offer a substantial throughput improvement, but it may not for
all cases. Particularly noticable for larger buffer sends/recvs.
See http://people.freebsd.org/~bmilekic/code/measure2.txt for a rough
idea.