virtio-net: remove xdp related info from page_to_skb()

For the clear construction of xdp_buff, we remove the xdp processing
interleaved with page_to_skb(). Now, the logic of xdp and building
skb from xdp are separate and independent.

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>