Rafał found an issue that for non-Ethernet interface, if we down and up
frequently, the memory will be consumed slowly.

The reason is we add allnodes/allrouters addressed in multicast list in
ipv6_add_dev(). When link down, we call ipv6_mc_down(), store all multicast
addresses via mld_add_delrec(). But when link up, we don't call ipv6_mc_up()
for non-Ethernet interface to remove the addresses. This makes idev->mc_tomb
getting bigger and bigger. The call stack looks like:

addrconf_notify(NETDEV_REGISTER)
	ipv6_add_dev
		ipv6_dev_mc_inc(ff01::1)
		ipv6_dev_mc_inc(ff02::1)
		ipv6_dev_mc_inc(ff02::2)

addrconf_notify(NETDEV_UP)
	addrconf_dev_config
		/* Alas, we support only Ethernet autoconfiguration. */
		return;

addrconf_notify(NETDEV_DOWN)
	addrconf_ifdown
		ipv6_mc_down
			igmp6_group_dropped(ff02::2)
				mld_add_delrec(ff02::2)
			igmp6_group_dropped(ff02::1)
			igmp6_group_dropped(ff01::1)

After investigating, I can't found a rule to disable multicast on
non-Ethernet interface. In RFC2460, the link could be Ethernet, PPP, ATM,
tunnels, etc. In IPv4, it doesn't check the dev type when calls ip_mc_up()
in inetdev_event(). Even for IPv6, we don't check the dev type and call
ipv6_add_dev(), ipv6_dev_mc_inc() after register device.

So I think it's OK to fix this memory consumer by calling ipv6_mc_up() for
non-Ethernet interface.

v2: Also check IFF_MULTICAST flag to make sure the interface supports
    multicast
Reported-by: Rafał Miłecki <zajec5@gmail.com>
Tested-by: Rafał Miłecki <zajec5@gmail.com>
Fixes: 74235a25 ("[IPV6] addrconf: Fix IPv6 on tuntap tunnels")
Fixes: 1666d49e ("mld: do not remove mld souce list info when set link down")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>