nfp: bpf: fix context accesses

Sizes of fields in struct xdp_md/xdp_buff and some in sk_buff depend
on target architecture.  Take that into account and use struct xdp_buff,
not struct xdp_md.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>