skbuff: make __skb_header_pointer()'s data argument const

The function never modifies the input buffer, so 'data' argument
can be marked as const.
This implies one harmless cast-away.
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -3678,11 +3678,11 @@ __wsum skb_checksum(const struct sk_buff *skb, int offset, int len,
 		    __wsum csum);
 
 static inline void * __must_check
-__skb_header_pointer(const struct sk_buff *skb, int offset,
-		     int len, void *data, int hlen, void *buffer)
+__skb_header_pointer(const struct sk_buff *skb, int offset, int len,
+		     const void *data, int hlen, void *buffer)
 {
 	if (hlen - offset >= len)
-		return data + offset;
+		return (void *)data + offset;
 
 	if (!skb ||
 	    skb_copy_bits(skb, offset, buffer, len) < 0)