virtio_net: introduce virtnet_xdp_handler() to seprate the logic of run xdp

At present, we have two similar logic to perform the XDP prog.

Therefore, this patch separates the code of executing XDP, which is
conducive to later maintenance.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>