tun: remove unnecessary tun_xdp_hdr structure

With f95f0f95("net, xdp: Introduce xdp_init_buff utility routine"),
buffer length could be stored as frame size so there's no need to have
a dedicated tun_xdp_hdr structure. We can simply store virtio net
header instead.

Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://patch.msgid.link/20250701010352.74515-1-jasowang@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>