ipv6: add missing netconf notif when 'all' is updated

The 'default' value was not advertised.

Fixes: f3a1bfb1

 ("rtnl/ipv6: use netconf msg to advertise forwarding status")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>