routing: move route expiration time to its nexthop

Expiration time is actually a path property, not a route property.
Move its storage to nexthop to simplify upcoming nhop(9) KPI changes
and netlink introduction.

Differential Revision: https://reviews.freebsd.org/D35970
MFC after: 2 weeks

(cherry picked from commit 2717e958df537b2885fdf42635d7b9dc793719b2)