hyperv/hn: Move TSO packet fixup to an earlier place for if_transmit.

While TSO packet header may be still cache-hot.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D8393

hyperv/hn: Directly fill chimney sending buffer for small packets.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D8394