virtio-net: don't do header check for dodgy gso packets

There's no need to do header check for virtio-net since:

- Host sets dodgy for all gso packets from guest and check the header.
- Host should be prepared for all kinds of evil packets from guest, since
  malicious guest can send any kinds of packet.

So this patch sets NETIF_F_GSO_ROBUST for virtio-net to skip the check.

Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>