ethernet: constify eth_get_headlen()'s data argument

It's used only for flow dissection, which now takes constant data
pointers.
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/etherdevice.h

@@ -29,7 +29,7 @@ struct device;
 int eth_platform_get_mac_address(struct device *dev, u8 *mac_addr);
 unsigned char *arch_get_platform_mac_address(void);
 int nvmem_get_mac_address(struct device *dev, void *addrbuf);
-u32 eth_get_headlen(const struct net_device *dev, void *data, unsigned int len);
+u32 eth_get_headlen(const struct net_device *dev, const void *data, u32 len);
 __be16 eth_type_trans(struct sk_buff *skb, struct net_device *dev);
 extern const struct header_ops eth_header_ops;
 

net/ethernet/eth.c

@@ -122,7 +122,7 @@ EXPORT_SYMBOL(eth_header);
  * Make a best effort attempt to pull the length for all of the headers for
  * a given frame in a linear buffer.
  */
-u32 eth_get_headlen(const struct net_device *dev, void *data, unsigned int len)
+u32 eth_get_headlen(const struct net_device *dev, const void *data, u32 len)
 {
 	const unsigned int flags = FLOW_DISSECTOR_F_PARSE_1ST_FRAG;
 	const struct ethhdr *eth = (const struct ethhdr *)data;