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/net/core/dev.c b/net/core/dev.c
index c2f3d5d..d91dfbe 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1741,7 +1741,7 @@
 			__net_timestamp(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)
 {
 	unsigned int len;