batman-adv: use ETH_HLEN instead of sizeof(struct ethhdr)

Instead of using sizeof(struct ethhdr) it is strongly recommended to use the
kernel macro ETH_HLEN. This patch substitute each occurrence of the former
expressione with the latter one.

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
diff --git a/net/batman-adv/icmp_socket.c b/net/batman-adv/icmp_socket.c
index b87518e..2e98a57 100644
--- a/net/batman-adv/icmp_socket.c
+++ b/net/batman-adv/icmp_socket.c
@@ -175,13 +175,13 @@
 	if (len >= sizeof(struct icmp_packet_rr))
 		packet_len = sizeof(struct icmp_packet_rr);
 
-	skb = dev_alloc_skb(packet_len + sizeof(struct ethhdr));
+	skb = dev_alloc_skb(packet_len + ETH_HLEN);
 	if (!skb) {
 		len = -ENOMEM;
 		goto out;
 	}
 
-	skb_reserve(skb, sizeof(struct ethhdr));
+	skb_reserve(skb, ETH_HLEN);
 	icmp_packet = (struct icmp_packet_rr *)skb_put(skb, packet_len);
 
 	if (copy_from_user(icmp_packet, buff, packet_len)) {