if_vtnet - Disable rx csum offload due to unsupported ipv6 rx csum offload.

if_vtnet - Disable rx csum offload due to unsupported ipv6 rx csum offload.

* Ignoring the checksum offloading in the receive path of the driver isn't
  sufficient, since we might receive only partially checksummed packets
  from the host.

* Unfortunately there is only a single feature flag for both ipv4 and ipv6
  receive checksum offloading, so we need to disable both for now.

* At the moment we don't support a way to explicitly enable the rx csum
  feature at runtime, but this will be easily possible by adding support
  for the VIRTIO_NET_F_CTRL_GUEST_OFFLOADS feature.

* Mention this as a caveat in the manpage.

* Update correct default value of hw.vtnet.lro_disable tunable in the
  manpage, to match the code again.