pmtud + ipnat RELENG_6_2 appears to be broken

[Pete French]

> All zeros! very interesting - am surprised the switch didn't kick up
> a fuss about that. Well, patch applied and rebooting... 

...and now all my outgoing packets have the correct MAC address as expected
on them. I alos notice that I am now only seeing packets destined for the
appropriate machine comming out of my switch. Before the patch I could see
all the packets for all machine using tcpdup.

-pete.