net: constify is_skb_forwardable's arguments

is_skb_forwardable is not supposed to change anything so constify its
arguments

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 934ca86..52914a8 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -3264,7 +3264,8 @@
 				    struct netdev_queue *txq, int *ret);
 int __dev_forward_skb(struct net_device *dev, struct sk_buff *skb);
 int dev_forward_skb(struct net_device *dev, struct sk_buff *skb);
-bool is_skb_forwardable(struct net_device *dev, struct sk_buff *skb);
+bool is_skb_forwardable(const struct net_device *dev,
+			const struct sk_buff *skb);
 
 extern int		netdev_budget;