rtnetlink: Fix missing error code in rtnl_bridge_notify()

The error code is missing in this code scenario, add the error code
'-EINVAL' to the return value 'err'.

Eliminate the follow smatch warning:

net/core/rtnetlink.c:4834 rtnl_bridge_notify() warn: missing error code
'err'.
Reported-by: NAbaci Robot <abaci@linux.alibaba.com>
Signed-off-by: NJiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -4842,8 +4842,10 @@ static int rtnl_bridge_notify(struct net_device *dev)
 	if (err < 0)
 		goto errout;
 
-	if (!skb->len)
+	if (!skb->len) {
+		err = -EINVAL;
 		goto errout;
+	}
 
 	rtnl_notify(skb, net, 0, RTNLGRP_LINK, NULL, GFP_ATOMIC);
 	return 0;