rtnetlink: change rtnl_stats_dump() return value

By returning 0 (or an error) instead of skb->len,
we allow NLMSG_DONE to be appended to the current
skb at the end of a dump, saving a couple of recvmsg()
system calls.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20240502113748.1622637-2-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/rtnetlink.c

@@ -6024,7 +6024,7 @@ static int rtnl_stats_dump(struct sk_buff *skb, struct netlink_callback *cb)
 	cb->args[1] = idx;
 	cb->args[0] = h;
 
-	return skb->len;
+	return err;
 }
 
 void rtnl_offload_xstats_notify(struct net_device *dev)