m_pkthdr.rcvif dangling pointer problem

[Ryan Stone]

I ran headlong into this issue today when trying to test some network
stack changes.  It's depressingly easy to crash HEAD by periodically
destroying vlan interfaces while you are sending traffic over those
interfaces -- I get a panic within minutes.

> http://people.freebsd.org/~glebius/patches/ifnet.no_free

This patch makes my test system survive longer but does not resolve the issue.