Fixed rtnetlink attributes packaging into netlink message.

I've also added NetworkLinkDel() function which allows deleting
of the existing network links.

Docker-DCO-1.1-Signed-off-by: Milos Gajdos <milosgajdos83@gmail.com> (github: milosgajdos83)
Replaced log.Fatalf() with t.Fatalf()

No need to import log package.

Docker-DCO-1.1-Signed-off-by: Milos Gajdos <milosgajdos83@gmail.com> (github: milosgajdos83)