Hi Devel,
There are very few sources out there for Netlink.
I am also unfamiliar with NETLINK_USERSOCK.
I am afraid I can't help you.
However, if you want to know more about obtaining route/interface information via Netlink.
You can try this webpage.
http://qos.ittc.ukans.edu/netlink/html/node1.html
Pertaining to this area, I have some experience and I will try my best to help you.
Daryl