IPv6: keep permanent addresses on admin down

Permanent IPV6 addresses should not be removed when the link is
set to admin down, only when device is removed.

When link is lost permanent addresses should be marked as tentative
so that when link comes back they are subject to duplicate address
detection (if DAD was enabled for that address).

Other routing systems keep manually configured IPv6 addresses
when link is set down.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>