Ip packet filtering with bridging on freebsd

[Darren Reed <avalon () COOMBS ANU EDU AU>]

If someone is doing packet filtering using ipfw to do packet filtering
with a FreeBSD box configured to do bridging, it is relatively easy to
make the box go "boom" as none of the standard header sanity checks
are done prior to the filter routine being called (check /sys/net/bridge.c)
It is a feature "copied" from OpenBSD but somehow large amounts of code
were not copied and bugs resulted.