sfc: handle XDP_TX outcomes of XDP eBPF programs

Provide an ndo_xdp_xmit function that uses the XDP tx queue for this
CPU to send the packet.

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