[RTNL]: Introduce the rtnl_kill_links helper.

This one is responsible for calling ->dellink on each net
device found in net to help with vlan net_exit hook in the
nearest future.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>