mbuf layout optimizations

http://people.freebsd.org/~jeff/mbuf2.diff

Hello,

This is a call for testers and feedback on my mbuf layout improvements. 
I'm trying to decide whether I will push to have these included in 8.0. 
After reducing the scope slightly from my last patch, I have not 
encountered any problems.  Kip Macy has also been using it for the past 
few weeks without issue.

You should not expect any functional changes from this patch.  The goal 
is mostly to pave the way fors more sensible mbuf handling in the future, 
although it does offer a few performance benefits.

The only issue is that cxgb support requires another set of patches from 
Kip.  If anyone needs those I will prod him to reply with that diff.

Thanks,
Jeff