Since ip_input() truncates the packet to ip->ip_len prior to entering the

Since ip_input() truncates the packet to ip->ip_len prior to entering the
protocol stack, ip_demux must incorporate an ip_len check in its tcp/udp
prechecks to avoid an assertion in the tcp/udp stacks if the packet is
malformed.

This is a temporary hack until Jeff and I can come up with a better way
to do per-protocol mbuf checks.  Basically the issue involved is that we
want to pull up the entire tcp/udp/etc... header in order to be able to
trivially demux an IP packet (choose which protocol thread to route it to).
Since we have to m_pullup the packet in the demux we would rather do all
header length checks in the demux as well so as not have to repeat them in
the protocol code.

Reported-by: Sven Willenberger <sven@dmv.com>
In-discussion-with: Jeffrey Hsu