arp: make arp_hdr_len() return unsigned int

Negative ARP header length are not a thing.

Constify arguments while I'm at it.

Space savings:

	add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-3 (-3)
	function                        old     new   delta
	arpt_do_table                  1163    1160      -3

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/if_arp.h b/include/linux/if_arp.h
index 3355efc..6756fea 100644
--- a/include/linux/if_arp.h
+++ b/include/linux/if_arp.h
@@ -31,7 +31,7 @@ static inline struct arphdr *arp_hdr(const struct sk_buff *skb)
 	return (struct arphdr *)skb_network_header(skb);
 }
 
-static inline int arp_hdr_len(struct net_device *dev)
+static inline unsigned int arp_hdr_len(const struct net_device *dev)
 {
 	switch (dev->type) {
 #if IS_ENABLED(CONFIG_FIREWIRE_NET)