ipv6: automatically enable stable privacy mode if stable_secret set

ipv6: automatically enable stable privacy mode if stable_secret set

Bjørn reported that while we switch all interfaces to privacy stable mode
when setting the secret, we don't set this mode for new interfaces. This
does not make sense, so change this behaviour.

Fixes: 622c81d57b392cc ("ipv6: generation of stable privacy addresses for link-local and autoconf")
Reported-by: Bjørn Mork <bjorn@mork.no>
Cc: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>