[rfc] UDP RSS awareness; handling IPv4 fragments

[Adrian Chadd]

Hi!

http://people.freebsd.org/~adrian/rss/20140802-rss-udp-ip-fragments-1.diff

This implements the RSS awareness bits for UDPv4, UDPv6 and IP
fragment handling. It should work for UDP transmit and receive.

There's a phabricator review: https://phabric.freebsd.org/D527

I'll finish off IPv6 RSS after this and then look at the various ways
that packets make their way in and out of the stack. There's a whole
lot of silliness with multicast and IP tunneling / IPSEC decapsulation
that requires a recompute of the hash.

I appreciate any/all feedback and testing! Especially testing!

Thanks!


-a