ipv6: addrconf: don't remove address state on ifdown if the address is being kept

Currently, addrconf_ifdown does not delete statically configured IPv6
addresses when the interface is brought down. The intent is that when
the interface comes back up the address will be usable again. However,
this doesn't actually work, because the system stops listening on the
corresponding solicited-node multicast address, so the address cannot
respond to neighbor solicitations and thus receive traffic. Also, the
code notifies the rest of the system that the address is being deleted
(e.g, RTM_DELADDR), even though it is not. Fix it so that none of this
state is updated if the address is being kept on the interface.

Tested: Added a statically configured IPv6 address to an interface,
started ping, brought link down, brought link up again. When link came
up ping kept on going and "ip -6 maddr" showed that the host was still
subscribed to there

Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>