samples/bpf: add option for native and skb mode for redirect apps

When testing with a driver that has both native and generic redirect support:

$ sudo ./samples/bpf/xdp_redirect -N 5 6
input: 5 output: 6
ifindex 6:    4961879 pkt/s
ifindex 6:    6391319 pkt/s
ifindex 6:    6419468 pkt/s

$ sudo ./samples/bpf/xdp_redirect -S 5 6
input: 5 output: 6
ifindex 6:    1845435 pkt/s
ifindex 6:    3882850 pkt/s
ifindex 6:    3893974 pkt/s

$ sudo ./samples/bpf/xdp_redirect_map -N 5 6
input: 5 output: 6
map[0] (vports) = 4, map[1] (map) = 5, map[2] (count) = 0
ifindex 6:    2207374 pkt/s
ifindex 6:    6212869 pkt/s
ifindex 6:    6286515 pkt/s

$ sudo ./samples/bpf/xdp_redirect_map -S 5 6
input: 5 output: 6
map[0] (vports) = 4, map[1] (map) = 5, map[2] (count) = 0
ifindex 6:    5052528 pkt/s
ifindex 6:    5736631 pkt/s
ifindex 6:    5739962 pkt/s

Signed-off-by: Andy Gospodarek <andy@greyhouse.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>