xen-netback: Minor refactoring of netback code

This patch contains a few bits of refactoring before introducing the grant
mapping changes:
- introducing xenvif_tx_pending_slots_available(), as this is used several
  times, and will be used more often
- rename the thread to vifX.Y-guest-rx, to signify it does RX work from the
  guest point of view

Signed-off-by: Zoltan Kiss <zoltan.kiss@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/xen-netback/interface.c b/drivers/net/xen-netback/interface.c
index 7669d49..bc32627 100644
--- a/drivers/net/xen-netback/interface.c
+++ b/drivers/net/xen-netback/interface.c
@@ -421,8 +421,8 @@
 		disable_irq(vif->rx_irq);
 	}
 
-	task = kthread_create(xenvif_kthread,
-			      (void *)vif, "%s", vif->dev->name);
+	task = kthread_create(xenvif_kthread_guest_rx,
+			      (void *)vif, "%s-guest-rx", vif->dev->name);
 	if (IS_ERR(task)) {
 		pr_warn("Could not allocate kthread for %s\n", vif->dev->name);
 		err = PTR_ERR(task);