xen-netback: making the bandwidth limiter runtime settable

With the current netback, the bandwidth limiter's parameters are only
settable during vif setup time.  This patch register a watch on them, and
thus makes them runtime changeable.

When the watch fires, the timer is reset.  The timer's mutex is used for
fencing the change.

Cc: Anthony Liguori <aliguori@amazon.com>
Signed-off-by: Imre Palik <imrep@amazon.de>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
index cab9f52..2c25e8e 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -641,7 +641,7 @@
 	queue->remaining_credit = min(max_credit, max_burst);
 }
 
-static void tx_credit_callback(unsigned long data)
+void xenvif_tx_credit_callback(unsigned long data)
 {
 	struct xenvif_queue *queue = (struct xenvif_queue *)data;
 	tx_add_credit(queue);
@@ -1169,8 +1169,6 @@
 	if (size > queue->remaining_credit) {
 		queue->credit_timeout.data     =
 			(unsigned long)queue;
-		queue->credit_timeout.function =
-			tx_credit_callback;
 		mod_timer(&queue->credit_timeout,
 			  next_credit);
 		queue->credit_window_start = next_credit;