[ARP]: Introduce the arp_hdr_len helper.

There are some place, that calculate the ARP header length. These
calculations are correct, but 
 a) some operate with "magic" constants,
 b) enlarge the code length (sometimes at the cost of coding style),
 c) are not informative from the first glance.

The proposal is to introduce a helper, that includes all the good
sides of these calculations.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/if_arp.h b/include/linux/if_arp.h
index 296e8e8..4d34018 100644
--- a/include/linux/if_arp.h
+++ b/include/linux/if_arp.h
@@ -156,6 +156,12 @@
 {
 	return (struct arphdr *)skb_network_header(skb);
 }
+
+static inline int arp_hdr_len(struct net_device *dev)
+{
+	/* ARP header, plus 2 device addresses, plus 2 IP addresses. */
+	return sizeof(struct arphdr) + (dev->addr_len + sizeof(u32)) * 2;
+}
 #endif
 
 #endif	/* _LINUX_IF_ARP_H */