virtio_net: fixing XDP for fully checksummed packets handling

The XDP program can't correctly handle partially checksummed
packets, but works fine with fully checksummed packets. If the
device has already validated fully checksummed packets, then
the driver doesn't need to re-validate them, saving CPU resources.

Additionally, the driver does not drop all partially checksummed
packets when VIRTIO_NET_F_GUEST_CSUM is not negotiated. This is
not a bug, as the driver has always done this.

Fixes: 436c9453

 ("virtio-net: keep vnet header zeroed after processing XDP")
Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>