batman-adv: Force mac header to start of data on xmit

The caller of ndo_start_xmit may not already have called skb_reset_mac_header. The returned value of skb_mac_header/eth_hdr therefore can be in the wrong position and even outside the current skbuff. This for example happens when the user binds to the device using a PF_PACKET-SOCK_RAW with enabled qdisc-bypass:

  int opt = 4;
  setsockopt(sock, SOL_PACKET, PACKET_QDISC_BYPASS, &opt, sizeof(opt));

Since eth_hdr is used all over the codebase, the batadv_interface_tx function must always take care of resetting it.

---
Cc: [email protected]

---
 net/batman-adv/soft-interface.c | 2 ++
 1 file changed, 2 insertions(+)

index 5db5a0a4..b85ca809 100644
@@ -221,6 +221,8 @@ static netdev_tx_t batadv_interface_tx(struct sk_buff *skb,
 
 	netif_trans_update(soft_iface);
 	vid = batadv_get_vid(skb, 0);
+
+	skb_reset_mac_header(skb);
 	ethhdr = eth_hdr(skb);
 
 	switch (ntohs(ethhdr->h_proto)) {

2.19.2