[IPV6]: Event type in addrconf_ifdown is mis-used.

addrconf_ifdown is broken in respect to the usage of how
parameter. This function is called with (event != NETDEV_DOWN) and (2)
on the IPv6 stop.  It the latter case inet6_dev from loopback device
should be destroyed.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>