samples/bpf: Decrement ttl in fib forwarding example

Only consider forwarding packets if ttl in received packet is > 1 and
decrement ttl before handing off to bpf_redirect_map.

Signed-off-by: David Ahern <dsahern@gmail.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>