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

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