[IPV6]: anycast refcnt fix

This patch fixes a bug in Linux IPv6 stack which caused anycast address
to be added to a device prior DAD has been completed. This led to
incorrect reference count which resulted in infinite wait for
unregister_netdevice completion on interface removal.

Signed-off-by: Michal Wrobel <xmxwx@asn.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>