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Fix failure to add an interface prefix route when route with the same

Description

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