ipv6, token: allow for clearing the current device token

The original tokenized iid support implemented via f53adae4 ("net: ipv6:
add tokenized interface identifier support") didn't allow for clearing a
device token as it was intended that this addressing mode was the only one
active for globally scoped IPv6 addresses. Later we relaxed that restriction
via 617fe29d ("net: ipv6: only invalidate previously tokenized addresses"),
and we should also allow for clearing tokens as there's no good reason why
it shouldn't be allowed.

Fixes: 617fe29d ("net: ipv6: only invalidate previously tokenized addresses")
Reported-by: Robin H. Johnson <robbat2@gentoo.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>