samples/bpf: xdp_fwd explain bpf_fib_lookup return codes

Make it clear that this XDP program depend on the network
stack to do the ARP resolution.  This is connected with the
BPF_FIB_LKUP_RET_NO_NEIGH return code from bpf_fib_lookup().

Another common mistake (seen via XDP-tutorial) is that users
don't realize that sysctl net.ipv{4,6}.conf.all.forwarding
setting is honored by bpf_fib_lookup.

Reported-by: Anton Protopopov <[email protected]>
Signed-off-by: Jesper Dangaard Brouer <[email protected]>
Reviewed-by: David Ahern <[email protected]>
Acked-by: Yonghong Song <[email protected]>
Reviewed-by: Toke Høiland-Jørgensen <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>

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