routing: split nexthop creation and rtentry creation.

This change is required for the upcoming introduction of the next
nexhop-based operations KPI, as it will create rtentry and nexthops
at different stages of route table modification.

Differential Revision: https://reviews.freebsd.org/D36072
MFC after: 2 weeks

(cherry picked from commit 66230639ce311c9fbc3a92e7039b8577a7577b6e)