Review needed: Mbuf double-free detection patch

[Petri Helenius]

> 
> [on a side note, it is a bit depressing to see the same
> code replicated twice, in m_free() and m_freem(). Couldn't
> one try to make m_freem() just call m_free() in a loop and
> save some code bloat ? I doubt the extra function call
> would harm performance too much.]
> 
Allocating / freeing mbufs is quite expensive already so it probably does not make 
that huge a difference. However the direction should be to improve the perfomance.

Pete