pmtud + ipnat RELENG_6_2 appears to be broken

[Pete French]

> Please test this attached patch, note it includes the previous change
> too.

I just tested this, and it lets me enable jumbo frames on the lagg
interface as well as fixing the previous zero MAC problem as
well. Currently running one of our production machines on it as a
test (in a farm so it doesnt matter too much if it fails).

-pete.