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 parti

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.

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vtnet.4: Fix SYNOPSIS ("device if_vtnet" -> "device vtnet").

vtnet.4: Use .Mt for email addresses.

vtnet.4: Add a reference to ifmedia(4).

Importing slightly modified virtio-net by vsrinivas

* TSO and LRO are off for now by default.

* Makefile changes. Adapting manpages. Fixing a typo in vtnet_tx_offload.

* Allocating tx headers with

Importing slightly modified virtio-net by vsrinivas

* TSO and LRO are off for now by default.

* Makefile changes. Adapting manpages. Fixing a typo in vtnet_tx_offload.

* Allocating tx headers with contigmalloc during attach instead of using
kmalloc in the transmit path.
In the worst case we need one tx header for two virtqueue ring entries.

* Additionally some whitespace fixes

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