netlink: fixup regression in RTM_GETADDR

This commit fixes a regression in AF_INET/RTM_GETADDR and
AF_INET6/RTM_GETADDR.

Before this commit, the kernel would stop dumping addresses once the first
skb was full and end the stream with NLMSG_DONE(-EMSGSIZE). The error
shouldn't be sent back to netlink_dump so the callback is kept alive. The
userspace is expected to call back with a new empty skb.

Changes from V1:
 - The error is not handled in netlink_dump anymore but rather in
   inet_dump_ifaddr and inet6_dump_addr directly as suggested by
   David Ahern.

Fixes: d7e38611 ("net/ipv4: Put target net when address dump fails due to bad attributes")
Fixes: 242afaa6 ("net/ipv6: Put target net when address dump fails due to bad attributes")

Cc: David Ahern <dsahern@gmail.com>
Cc: "David S . Miller" <davem@davemloft.net>
Cc: netdev@vger.kernel.org
Signed-off-by: Arthur Gautier <baloo@gandi.net>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/devinet.c

@@ -1826,7 +1826,7 @@ static int inet_dump_ifaddr(struct sk_buff *skb, struct netlink_callback *cb)
 	if (fillargs.netnsid >= 0)
 		put_net(tgt_net);
 
-	return err < 0 ? err : skb->len;
+	return skb->len ? : err;
 }
 
 static void rtmsg_ifa(int event, struct in_ifaddr *ifa, struct nlmsghdr *nlh,

net/ipv6/addrconf.c

@@ -5154,7 +5154,7 @@ static int inet6_dump_addr(struct sk_buff *skb, struct netlink_callback *cb,
 	if (fillargs.netnsid >= 0)
 		put_net(tgt_net);
 
-	return err < 0 ? err : skb->len;
+	return skb->len ? : err;
 }
 
 static int inet6_dump_ifaddr(struct sk_buff *skb, struct netlink_callback *cb)