iproute2 5.1+ doesn't allow prefixlen!=32 in get

In order to set up routing exclusions properly, we now ask for routes without specified prefixlen and later add it manually before calling ip route add. Older versions of ip silently ignored the prefixlen -- as confirmed in: https://www.spinics.net/lists/netdev/msg570803.html.

Reproduction:

$ ip route get 1.2.0.0/24
Error: ipv4: Invalid values in header for route get request.