netpoll: prevent netpoll setup on slave devices

In commit 8d8fc29d
(netpoll: disable netpoll when enslave a device), we automatically
disable netpoll when the underlying device is being enslaved,
we also need to prevent people from setuping netpoll on
devices that are already enslaved.
Signed-off-by: NWANG Cong <amwang@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/netpoll.c

@@ -792,6 +792,12 @@ int netpoll_setup(struct netpoll *np)
 		return -ENODEV;
 	}
 
+	if (ndev->master) {
+		printk(KERN_ERR "%s: %s is a slave device, aborting.\n",
+		       np->name, np->dev_name);
+		return -EBUSY;
+	}
+
 	if (!netif_running(ndev)) {
 		unsigned long atmost, atleast;