Receive Segment Coalescing (RSC) in the vSwitch is a feature available in
Windows Server 2019 hosts and later. It reduces the per packet processing
overhead by coalescing multiple TCP segments when possible. This happens
mostly when TCP traffics are among different guests on same host.
This patch adds netvsc driver support for this feature.

The patch also updates NVS version to 6.1 as needed for RSC
enablement.

MFC after:	2 weeks
Sponsored by:	Microsoft
Differential Revision:	https://reviews.freebsd.org/D29075