batman-adv: use vlan_/eth_hdr() instead of skb->data in interface_tx path

Our .ndo_start_xmit handler (batadv_interface_tx()) can rely on having
the skb mac header pointer set correctly since the following commit
present in kernels >= 3.9:

"net: reset mac header in dev_start_xmit()" (6d1ccff627)

Therefore we can safely use eth_hdr() and vlan_eth_hdr() instead of
skb->data now, which spares us some ugly type casts.

At the same time set the mac_header in batadv_dat_snoop_incoming_arp_request()
before sending the skb along the TX path.

Signed-off-by: Linus Lüssing <linus.luessing@web.de>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>
diff --git a/net/batman-adv/distributed-arp-table.c b/net/batman-adv/distributed-arp-table.c
index edee504..5bb37a8 100644
--- a/net/batman-adv/distributed-arp-table.c
+++ b/net/batman-adv/distributed-arp-table.c
@@ -1027,6 +1027,11 @@
 	if (!skb_new)
 		goto out;
 
+	/* the rest of the TX path assumes that the mac_header offset pointing
+	 * to the inner Ethernet header has been set, therefore reset it now.
+	 */
+	skb_reset_mac_header(skb_new);
+
 	if (vid & BATADV_VLAN_HAS_TAG)
 		skb_new = vlan_insert_tag(skb_new, htons(ETH_P_8021Q),
 					  vid & VLAN_VID_MASK);