[IPV6]: Address autoconfiguration does not work after device down/up cycle

If you set network interface down and up again, the IPv6 address
autoconfiguration does not work. 'ip addr' shows that the link-local
address is in tentative state. We don't even react to periodical router
advertisements.

During NETDEV_DOWN we clear IF_READY, and we don't set it back in
NETDEV_UP. While starting to perform DAD on the link-local address, we
notice that the device is not in IF_READY, and we abort autoconfiguration
process (which would eventually send router solicitations).
Acked-by: Juha-Matti Tapio <jmtapio@verkkotelakka.net>
Acked-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -2165,6 +2165,9 @@ static int addrconf_notify(struct notifier_block *this, unsigned long event,
 					dev->name);
 				break;
 			}
+
+			if (idev)
+				idev->if_flags |= IF_READY;
 		} else {
 			if (!netif_carrier_ok(dev)) {
 				/* device is still not ready. */