rtnetlink: use the new rtnl_get_event() interface

Small clean-up to rtmsg_ifinfo() to use the rtnl_get_event()
interface instead of using 'internal' values directly.

Signed-off-by: Vladislav Yasevich <vyasevic@redhat.com>
Acked-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>