Kernel requires that the host part of a route (based on network/plen)
is zero. Routes with non-zero host part don't really exist.

In settings (NMIPRoute), we don't enforce that. Hence we must ensure
that we don't let such invalid routes into NMIP4Config/NMIP6Config.

Also at other places where we obtain routes from untrusted sources,
we must sanitize them first.

Also add an assertion to catch such bugs.