virtio_net: replace VIRTIO_XDP_HEADROOM by XDP_PACKET_HEADROOM

virtio_net: replace VIRTIO_XDP_HEADROOM by XDP_PACKET_HEADROOM

virtio net has VIRTIO_XDP_HEADROOM that is equal to
XDP_PACKET_HEADROOM to calculate the headroom for xdp.

But here we should use the macro XDP_PACKET_HEADROOM from bpf.h to
calculate the headroom for xdp. So here we remove the
VIRTIO_XDP_HEADROOM, and use the XDP_PACKET_HEADROOM to replace it.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://patch.msgid.link/20240708112537.96291-2-xuanzhuo@linux.alibaba.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>