[NET]: Make and use skb_get_queue_mapping

Make the helper for getting the field, symmetrical to
the "set" one. Return 0 if CONFIG_NETDEVICES_MULTIQUEUE=n

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/cpmac.c b/drivers/net/cpmac.c
index ed53aaa..ae41973 100644
--- a/drivers/net/cpmac.c
+++ b/drivers/net/cpmac.c
@@ -471,7 +471,7 @@
 	}
 
 	len = max(skb->len, ETH_ZLEN);
-	queue = skb->queue_mapping;
+	queue = skb_get_queue_mapping(skb);
 #ifdef CONFIG_NETDEVICES_MULTIQUEUE
 	netif_stop_subqueue(dev, queue);
 #else