mbuf trash-related "memory modified after free" panic fixed.

Rev 1.9 of kern_mbuf.c contains a fix for the "memory modified after free" 
panics that have been cropping up recently.  Bosko Milekic looked over 
what I had committed last week and found that I had forgotten one very 
important line of code.

So, if you've been seeing such panics, please cvsup (or manually grab rev 
1.9) and rebuild your kernel.  If you're still seeing memory modified 
after free panics/prints after this upgrade, I definitely want to hear 
about it.

Thanks, and sorry for panics.

Mike "Silby" Silbersack