arp: fill cache only from ARP responses

[Dirbaio]

This fill here fills the cache on any ARP message. It should only do it on ARP responses I think.

smoltcp/src/iface/interface.rs

Line 1044 in db18514

self.neighbor_cache.as_mut().unwrap().fill(

[dnadlinger]

Isn't opportunistically use everything it sees to fill the cache a sensible thing to do?

[Dirbaio]

@dnadlinger In theory it's correct, but I'm quite scared that some broken network out there breaks it, similarly to what happened in 6210612. Addresses in ARP packets should be more reliable yes, but still, who knows...

Also it can fill the the ARP cache with random entries we don't need, which can evict the ones that we do need. So this change can be good for performance too.