Reorganise nd6 notification code to avoid direct rtentry field access.

[melifaro]

Summary

One of the goals of the new routing KPI defined in r359823 is to entirely hide`struct rtentry` from the consumers.
It will allow to improve routing subsystem internals and deliver more features much faster.
This change is one of the ongoing changes to eliminate direct struct rtentry field accesses.

In particular, it reorganises nd6 notification code to avoid accessing most of the rtentry fields.