Fix failure to add an interface prefix route when route with the same
prefix is already presented in the routing table.
PR: 277125
Reported by: Oleksandr Ignatyev <alex@i.org.ua>
Reviewed by: ae, jlduran
Tested by: jlduran
Differential Revision: https://reviews.freebsd.org/D47534
MFC after: 2 weeks