net: stmmac: introduce wrapper for struct xdp_buff

Introduce struct stmmac_xdp_buff as a preparation to support XDP Rx
metadata via kfuncs.

Signed-off-by: Song Yoong Siang <yoong.siang.song@intel.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>