[rfc] UDP RSS transmit/receive message options; ip_output() inp flowid changes

[Adrian Chadd]

Hi!

I'd appreciate a review of this:

http://people.freebsd.org/~adrian/rss/20140722-rss-udp-1.diff

The overview:

* Add a new flag to ip_output() to instruct it to not override the
flowid with the inp cached value. Some forms of UDP transmit will
break this.
* Add new IP socket options to receive flowid and rss bucket
information with the UDP datagram (via recvmsg.)
* Add awareness in the send path of those same messages so the
transmit path can avoid doing a hash calculation.

The software hash calculations aren't done yet in the UDP transmit
path - I'll work on those next.

There are some comments which won't make it into the final committed
patch. Bonus points if you get this far and see what they are.



-a