MFC r264838:

Don't free an mbuf twice. This only happens in very rare error
cases where the peer sends illegal sequencing information in
DATA chunks for an existing association.
MFC r265455:

Remove unused code. This is triggered by the bugreport of Sylvestre Ledru
which deal with useless code in the user land stack:
https://bugzilla.mozilla.org/show_bug.cgi?id=1003929
MFC r265691, r265713:

For some UDP packets (for example with 200 byte payload) and IP options,
the IP header and the UDP header are not in the same mbuf.
Add code to in_delayed_cksum() to deal with this case.

Use KASSERTs as suggested by glebius@