sunvnet: add support for IPv6 checksum offloads

The original code didn't handle non-IPv4 packets very well, so the
offload advertising had to be scaled back down to just IP.  Here we
add the bits needed to support TCP and UDP packets over IPv6 and
turn the offload advertising back on.

Orabug: 26289579

Signed-off-by: Shannon Nelson <shannon.nelson@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>