hyperv/hn: Avoid mbuf cluster allocation, if the packet is small.

This one mainly avoids mbuf cluster allocation for TCP ACKs during
TCP sending tests. And it gives me ~200Mbps improvement (4.7Gbps
-> 4.9Gbps), when running iperf3 TCP sending test w/ 16 connections.

While I'm here, nuke the unnecessary zeroing out pkthdr.csum_flags.

Reviewed by: adrain
Approved by: adrian (mentor)
Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D4853