hyperv/hn: Enable IP header checksum offloading

So that:

• TCP/IP stack will not do unnecessary IP header checksum for TSO packets.
• Reduce guest load for non-TSO IP packets.

Reviewed by: adrian
Approved by: adrian (mentor)
MFC after: 1 week
Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D5099