One of the goals of the new routing KPI defined in r359823 is to entirely hide`struct rtentry` from the consumers. It will allow to improve routing subsystem internals and deliver more features much faster.
This change is one of the final changes eliminating direct access to the `struct rtentry` fields.
In particular, it introduces new `rt_export_prefix()` function, exporting rtentry destination as sockaddr and mask as prefix length.