net: Properly unlink GRO packets on overflow.

Just like with normal GRO processing, we have to initialize
skb->next to NULL when we unlink overflow packets from the
GRO hash lists.

Fixes: d4546c2509 ("net: Convert GRO SKB handling to list_head.")
Reported-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -5457,7 +5457,7 @@ static void gro_flush_oldest(struct list_head *head)
 	/* Do not adjust napi->gro_hash[].count, caller is adding a new
 	 * SKB to the chain.
 	 */
-	list_del(&oldest->list);
+	skb_list_del_init(oldest);
 	napi_gro_complete(oldest);
 }