Reorganise nd6 notification code to avoid direct rtentry field access.

One of the goals of the new routing KPI defined in r359823 is to entirely hide
struct rtentry from the consumers. Doing so will allow to improve routing
subsystem internals and deliver features more easily. This change is one of

the ongoing changes to eliminate direct struct rtentry field accesses.

It introduces rtfree_func() wrapper around RTFREE() and reorganises nd6 notification
code to avoid accessing most of the rtentry fields.

Reviewed by: ae
Differential Revision: https://reviews.freebsd.org/D24404