ndisc: do not Unrecognized packets with unknown options

[Dirbaio]

ndisc is throwing Unrecognized when seeing unknown options, for example here

https://github.com/smoltcp-rs/smoltcp/blob/35e833e33dfd3e4efc3eb7d5de06bec17c54b011/src/wire/ndisc.rs#L239

but rfc4861 section 9 says

In order to ensure that future extensions properly coexist with current implementations, all nodes MUST silently ignore any options they do not recognize in received ND packets and continue processing the packet.