net: ipv6: Do not keep IPv6 addresses when IPv6 is disabled

If IPv6 is disabled when the option is set to keep IPv6
addresses on link down, userspace is unaware of this as
there is no such indication via netlink. The solution is to
remove the IPv6 addresses in this case, which results in
netlink messages indicating removal of addresses in the
usual manner. This fix also makes the behavior consistent
with the case of having IPv6 disabled first, which stops
IPv6 addresses from being added.

Fixes: f1705ec1 ("net: ipv6: Make address flushing on ifdown optional")
Signed-off-by: NMike Manning <mmanning@brocade.com>
Acked-by: NDavid Ahern <dsa@cumulusnetworks.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -3543,7 +3543,7 @@ static int addrconf_ifdown(struct net_device *dev, int how)
 	/* combine the user config with event to determine if permanent
 	 * addresses are to be removed from address hash table
 	 */
-	keep_addr = !(how || _keep_addr <= 0);
+	keep_addr = !(how || _keep_addr <= 0 || idev->cnf.disable_ipv6);
 
 	/* Step 2: clear hash table */
 	for (i = 0; i < IN6_ADDR_HSIZE; i++) {
@@ -3599,7 +3599,7 @@ static int addrconf_ifdown(struct net_device *dev, int how)
 	/* re-combine the user config with event to determine if permanent
 	 * addresses are to be removed from the interface list
 	 */
-	keep_addr = (!how && _keep_addr > 0);
+	keep_addr = (!how && _keep_addr > 0 && !idev->cnf.disable_ipv6);
 
 	INIT_LIST_HEAD(&del_list);
 	list_for_each_entry_safe(ifa, tmp, &idev->addr_list, if_list) {