Am I just being particularly dim today, or can the call to
dev->change_rx_flags(dev, IFF_MULTICAST) in dev_change_flags() never
happen?

We've just set dev->flags = flags & IFF_MULTICAST, effectively. So the
condition '(dev->flags ^ flags) & IFF_MULTICAST' is _never_ going to be
true.
Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>