ipv6: Fix address dump when IPv6 is disabled on an interface

Cited commit started returning an error when user space requests to dump
the interface's IPv6 addresses and IPv6 is disabled on the interface.
Restore the previous behavior and do not return an error.

Before cited commit:

 # ip address show dev dummy1
 2: dummy1: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN group default qlen 1000
     link/ether 1a:52:02:5a:c2:6e brd ff:ff:ff:ff:ff:ff
     inet6 fe80::1852:2ff:fe5a:c26e/64 scope link proto kernel_ll
        valid_lft forever preferred_lft forever
 # ip link set dev dummy1 mtu 1000
 # ip address show dev dummy1
 2: dummy1: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1000 qdisc noqueue state UNKNOWN group default qlen 1000
     link/ether 1a:52:02:5a:c2:6e brd ff:ff:ff:ff:ff:ff

After cited commit:

 # ip address show dev dummy1
 2: dummy1: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN group default qlen 1000
     link/ether 1e:9b:94:00:ac:e8 brd ff:ff:ff:ff:ff:ff
     inet6 fe80::1c9b:94ff:fe00:ace8/64 scope link proto kernel_ll
        valid_lft forever preferred_lft forever
 # ip link set dev dummy1 mtu 1000
 # ip address show dev dummy1
 RTNETLINK answers: No such device
 Dump terminated

With this patch:

 # ip address show dev dummy1
 2: dummy1: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN group default qlen 1000
     link/ether 42:35:fc:53:66:cf brd ff:ff:ff:ff:ff:ff
     inet6 fe80::4035:fcff:fe53:66cf/64 scope link proto kernel_ll
        valid_lft forever preferred_lft forever
 # ip link set dev dummy1 mtu 1000
 # ip address show dev dummy1
 2: dummy1: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1000 qdisc noqueue state UNKNOWN group default qlen 1000
     link/ether 42:35:fc:53:66:cf brd ff:ff:ff:ff:ff:ff

Fixes: 9cc4cc3 ("ipv6: use xa_array iterator to implement inet6_dump_addr()")
Reported-by: Gal Pressman <gal@nvidia.com>
Closes: https://lore.kernel.org/netdev/7e261328-42eb-411d-b1b4-ad884eeaae4d@linux.dev/
Tested-by: Gal Pressman <gal@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240321173042.2151756-1-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: idosch

net/ipv6/addrconf.c

@@ -5416,10 +5416,11 @@ static int inet6_dump_addr(struct sk_buff *skb, struct netlink_callback *cb,
 
 		err = 0;
 		if (fillargs.ifindex) {
-			err = -ENODEV;
 			dev = dev_get_by_index_rcu(tgt_net, fillargs.ifindex);
-			if (!dev)
+			if (!dev) {
+				err = -ENODEV;
 				goto done;
+			}
 			idev = __in6_dev_get(dev);
 			if (idev)
 				err = in6_dump_addrs(idev, skb, cb,