batman-adv: fix vlan header access

When batadv_get_vid() is invoked in interface_rx() the
batman-adv header has already been removed, therefore
the header_len argument has to be 0.

Introduced by c018ad3d
("batman-adv: add the VLAN ID attribute to the TT entry")
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>

net/batman-adv/soft-interface.c

@@ -351,7 +351,7 @@ void batadv_interface_rx(struct net_device *soft_iface,
 	 */
 	nf_reset(skb);
 
-	vid = batadv_get_vid(skb, hdr_size);
+	vid = batadv_get_vid(skb, 0);
 	ethhdr = eth_hdr(skb);
 
 	switch (ntohs(ethhdr->h_proto)) {