if_awg fixes

[Guy Yur]

Hi,

>>>> I had issues with if_awg when passing heavy traffic.
>>>> (transmission, samba, ...)
>>>> a. The tx hardware checksum stopped working after a while.
>>>> b. mbuf leaks.

(removed old discussion)

I split the tx path changes so hopefully they are now easier to review.
The changes were rebased after r325410 (latest if_awg modification).
https://reviews.freebsd.org/D13029
https://reviews.freebsd.org/D13030
https://reviews.freebsd.org/D13031
https://reviews.freebsd.org/D13032
https://reviews.freebsd.org/D13033
https://reviews.freebsd.org/D13034
https://reviews.freebsd.org/D13035
https://reviews.freebsd.org/D12538
https://reviews.freebsd.org/D12539

Thanks,
Guy