Maint: internal/routing IP rules functions

- Take in `src` as `*net.IPNet` instead of `net.IP`
- Take `dst` IP network
- Debug logged `ip rule` dynamically built
- Add unit tests for all IP rules functions

internal/netlink/interface.go

@@ -1,5 +1,7 @@
 package netlink
 
+//go:generate mockgen -destination=mock_$GOPACKAGE/$GOFILE . NetLinker
+
 var _ NetLinker = (*NetLink)(nil)
 
 type NetLinker interface {