xen-netback: fix a BUG() during initialization

Commit edafc132 ("xen-netback: making the bandwidth limiter runtime settable")
introduced the capability to change the bandwidth rate limit at runtime.
But it also introduced a possible crashing bug.

If netback receives two XenbusStateConnected without getting the
hotplug-status watch firing in between, then it will try to register the
watches for the rate limiter again.  But this triggers a BUG() in the watch
registration code.

The fix modifies connect() to remove the possibly existing packet-rate
watches before trying to install those watches.  This behaviour is in line
with how connect() deals with the hotplug-status watch.
Signed-off-by: Imre Palik <imrep@amazon.de>
Cc: Matt Wilson <msw@amazon.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netback/xenbus.c

@@ -681,6 +681,9 @@ static int xen_register_watchers(struct xenbus_device *dev, struct xenvif *vif)
 	char *node;
 	unsigned maxlen = strlen(dev->nodename) + sizeof("/rate");
 
+	if (vif->credit_watch.node)
+		return -EADDRINUSE;
+
 	node = kmalloc(maxlen, GFP_KERNEL);
 	if (!node)
 		return -ENOMEM;
@@ -770,6 +773,7 @@ static void connect(struct backend_info *be)
 	}
 
 	xen_net_read_rate(dev, &credit_bytes, &credit_usec);
+	xen_unregister_watchers(be->vif);
 	xen_register_watchers(dev, be->vif);
 	read_xenbus_vif_flags(be);