[Patch] netpoll: prevent netpoll setup on slave devices

[Amerigo Wang <amwang@redhat.com>]

In commit 8d8fc29d02a33e4bd5f4fa47823c1fd386346093
(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: WANG Cong <amwang@redhat.com>
Cc: Neil Horman <nhorman@tuxdriver.com>

---
diff --git a/net/core/netpoll.c b/net/core/netpoll.c
index 2d7d6d4..42ea4b0 100644
--- a/net/core/netpoll.c
+++ b/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;