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.