net: skbuff: const-ify casts in skb_queue_* functions

We should const-ify comparisons on skb_queue_* inline helper
functions as their parameters are const as well, so lets not
drop that.
Suggested-by: NBrad Spengler <spender@grsecurity.net>
Signed-off-by: NDaniel Borkmann <dborkman@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -827,7 +827,7 @@ static inline struct skb_shared_hwtstamps *skb_hwtstamps(struct sk_buff *skb)
  */
 static inline int skb_queue_empty(const struct sk_buff_head *list)
 {
-	return list->next == (struct sk_buff *)list;
+	return list->next == (const struct sk_buff *) list;
 }
 
 /**
@@ -840,7 +840,7 @@ static inline int skb_queue_empty(const struct sk_buff_head *list)
 static inline bool skb_queue_is_last(const struct sk_buff_head *list,
 				     const struct sk_buff *skb)
 {
-	return skb->next == (struct sk_buff *)list;
+	return skb->next == (const struct sk_buff *) list;
 }
 
 /**
@@ -853,7 +853,7 @@ static inline bool skb_queue_is_last(const struct sk_buff_head *list,
 static inline bool skb_queue_is_first(const struct sk_buff_head *list,
 				      const struct sk_buff *skb)
 {
-	return skb->prev == (struct sk_buff *)list;
+	return skb->prev == (const struct sk_buff *) list;
 }
 
 /**