batman-adv: update data pointers after skb_cow()

BugLink: https://bugs.launchpad.net/bugs/1868629

commit bc44b781 upstream.

batadv_check_unicast_ttvn() calls skb_cow(), so pointers into the SKB data
must be (re)set after calling it. The ethhdr variable is dropped
altogether.

Fixes: 78fc6bbe0aca ("batman-adv: add UNICAST_4ADDR packet type")
Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Kelsey Skunberg <kelsey.skunberg@canonical.com>

net/batman-adv/routing.c

@@ -904,7 +904,6 @@ int batadv_recv_unicast_packet(struct sk_buff *skb,
 	bool is4addr;
 
 	unicast_packet = (struct batadv_unicast_packet *)skb->data;
-	unicast_4addr_packet = (struct batadv_unicast_4addr_packet *)skb->data;
 
 	is4addr = unicast_packet->packet_type == BATADV_UNICAST_4ADDR;
 	/* the caller function should have already pulled 2 bytes */
@@ -925,9 +924,13 @@ int batadv_recv_unicast_packet(struct sk_buff *skb,
 	if (!batadv_check_unicast_ttvn(bat_priv, skb, hdr_size))
 		return NET_RX_DROP;
 
+	unicast_packet = (struct batadv_unicast_packet *)skb->data;
+
 	/* packet for me */
 	if (batadv_is_my_mac(bat_priv, unicast_packet->dest)) {
 		if (is4addr) {
+			unicast_4addr_packet =
+				(struct batadv_unicast_4addr_packet *)skb->data;
 			subtype = unicast_4addr_packet->subtype;
 			batadv_dat_inc_counter(bat_priv, subtype);