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
(cherry picked from commit 1da4954c92ea7585b352ba830d3ee64ca69ada52)