xdp: move struct xdp_buff from filter.h to xdp.h

This is done to prepare for the next patch, and it is also
nice to move this XDP related struct out of filter.h.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>