batman-adv: Fix ICMP RR ethernet access after skb_linearize

The skb_linearize may reallocate the skb. This makes the calculated pointer
for ethhdr invalid. But it the pointer is used later to fill in the RR
field of the batadv_icmp_packet_rr packet.

Instead re-evaluate eth_hdr after the skb_linearize+skb_cow to fix the
pointer and avoid the invalid read.

Fixes: da6b8c20a5b8 ("batman-adv: generalize batman-adv icmp packet handling")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/batman-adv/routing.c

@@ -374,6 +374,7 @@ int batadv_recv_icmp_packet(struct sk_buff *skb,
 		if (skb_cow(skb, ETH_HLEN) < 0)
 			goto out;
 
+		ethhdr = eth_hdr(skb);
 		icmph = (struct batadv_icmp_header *)skb->data;
 		icmp_packet_rr = (struct batadv_icmp_packet_rr *)icmph;
 		if (icmp_packet_rr->rr_cur >= BATADV_RR_LEN)