A couple times recently somebody has noticed that we're ignoring a
sequence number here and wondered whether there's a bug.
In fact, there's not. Thanks to Andy Adamson for pointing out a useful
explanation in rfc 2203. Add comments citing that rfc, and remove
"seqnum" to prevent static checkers complaining about unused variables.
Reported-by: Andi Kleen <andi@firstfloor.org> Signed-off-by: J. Bruce Fields <bfields@redhat.com>