Mbuf double-free guilty party detection patch

The attached patch stores the address of who freed an 
mbuf/cluster/whatever inside it, then prints that address when panicing. 
You can then feed that address into "x 0xwhatever" in DDB to see who the 
semi-guilty party is.

Two flaws in the patch as is:

- It's messy and not compatible with non-i386, cleanups are needed.

- If the mbuf in question is part of a mbuf chain, we'll see m_freem as 
the guilty party, because it called m_free.

So, if you're one of the people seeing panics due to mbufs being used 
after free, please try applying the patch and see what results you get. 
If you keep getting m_freem as the previous user, then I'll have to 
enhance it to work around that.

Mike "Silby" Silbersack