add_neighbour example: set required link_local_address

[kaueraal]

Adding (or replacing) a neighbour fails for me without setting a link local address. First, I receive error 22 and all reattempts receive error 17.

This e.g. also shows in the example. After building cargo build --example add_neighbour as a normal user I get the following when running it as root:

# ./add_neighbour eth0 10.17.31.55                                                                                                         
Received a netlink error message Invalid argument (os error 22)                                                                                                                     
# ./add_neighbour eth0 10.17.31.55                                                                                                         
Received a netlink error message File exists (os error 17) 

This is also in line with ip which also requires setting a link local address for me:

# ip neigh add 10.17.31.55 dev eth0
Error: No link layer address given.

Hence, I assume this is the correct behaviour. I don't know whether the example worked at some point and the behaviour in the kernel changed. I have cargo 1.78.0 (54d8815d0 2024-03-26), I run everything on a Debian 12 with kernel 6.1.76 and ip -V reports iproute2-6.1.0, libbpf 1.1.0.

To fix the example, I adjusted it in this MR to also get a link local address as argument:

# ./add_neighbour eth0 10.17.31.55 56:78:90:ab:cd:ef
Done