[RTNL]: Relax for_each_netdev_safe in __rtnl_link_unregister.

Each potential list_del (happening from inside a ->dellink call)
is followed by goto restart, so there's no need in _safe iteration.

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