net/ipv6: don't return positive numbers when nothing was dumped

in6_dump_addrs() returns a positive 1 if there was nothing to dump.
This return value can not be passed as return from inet6_dump_addr()
as is, because it will confuse rtnetlink, resulting in NLMSG_DONE
never getting set:

$ ip addr list dev lo
EOF on netlink
Dump terminated

v2: flip condition to avoid a new goto (DaveA)

Fixes: 7c1e8a38 ("netlink: fixup regression in RTM_GETADDR")
Reported-by: NBrendan Galloway <brendan.galloway@netronome.com>
Signed-off-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: NDavid Ahern <dsahern@gmail.com>
Tested-by: NDavid Ahern <dsahern@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -5120,6 +5120,8 @@ static int inet6_dump_addr(struct sk_buff *skb, struct netlink_callback *cb,
 			if (idev) {
 				err = in6_dump_addrs(idev, skb, cb, s_ip_idx,
 						     &fillargs);
+				if (err > 0)
+					err = 0;
 			}
 			goto put_tgt_net;
 		}