virtio-net: set up xdp for multi buffer packets

When the xdp program sets xdp.frags, which means it can process
multi-buffer packets over larger MTU, so we continue to support xdp.

Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>