[IPV6]: /proc/net/anycast6 unbalanced inet6_dev refcnt

Reading /proc/net/anycast6 when there is no anycast address
on an interface results in an ever-increasing inet6_dev reference
count, as well as a reference to the netdevice you can't get rid of.

Signed-off-by: David S. Miller <davem@davemloft.net>