[IPV4]: Convert address addition to new netlink api

Adds rtm_to_ifaddr() transforming a netlink message to a
struct in_ifaddr. Fixes various unvalidated netlink attributes
causing memory corruptions when left empty by userspace
applications.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>