carp: Fix routes reset issue

carp: Fix routes reset issue

Before this commit, routes related to carp or carp backing device were
reset, i.e. only prefix route was left, when carp state changed, e.g.
carp is down.

This commit fixes this routes reset issue by replacing the carp or carp
device routes' rt_ifa and rt_ifp with proper ifaddr and ifnet, instead
of discarding the old routes and installing the new prefix route.

Reported-by: robgar