ipv6 addrlabel: permit deletion of labels assigned to removed dev

as addrlabels with an interface index are left alone when the
interface gets removed this results in addrlabels that can no
longer be removed.

Restrict validation of index to adding new addrlabels.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>