network/ndisc: restore the original preference and priority before checking if existi...

network/ndisc: restore the original preference and priority before checking if existing route can be updated

Follow-up for 972f1d17ab461a51142a142609dd3ec50bae8440.

This fixes the logic of removing unnecessary routes configured by the
previously received RAs. Previously, we wrongly handled existing routes
could be updated, and unexpected routes would be kept.