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/send.c b/net/batman-adv/send.c
index b5f078c..7c66b61 100644
--- a/net/batman-adv/send.c
+++ b/net/batman-adv/send.c
@@ -51,7 +51,7 @@
 	}
 
 	/* push to the ethernet header. */
-	if (my_skb_head_push(skb, sizeof(*ethhdr)) < 0)
+	if (my_skb_head_push(skb, ETH_HLEN) < 0)
 		goto send_skb_err;
 
 	skb_reset_mac_header(skb);