netlink: Fix bugs in nlmsg_end() conversions.

Commit 053c095a82cf ("netlink: make nlmsg_end() and genlmsg_end()
void") didn't catch all of the cases where callers were breaking out
on the return value being equal to zero, which they no longer should
when zero means success.

Fix all such cases.

Reported-by: Marcel Holtmann <marcel@holtmann.org>
Reported-by: Scott Feldman <sfeldma@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index 8975d95..d6b4f5d 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -4213,7 +4213,7 @@
 				goto cont;
 
 			if (in6_dump_addrs(idev, skb, cb, type,
-					   s_ip_idx, &ip_idx) <= 0)
+					   s_ip_idx, &ip_idx) < 0)
 				goto done;
 cont:
 			idx++;