net, atm/ambassader: convert skb->tail into skb_tail_pointer(skb)

The change set of 27a884dc, "[SK_BUFF]: Convert skb->tail to sk_buff_data_t"
converted skb->tail from pointer into sk_buff_data_t. It missed skb->tail
in drivers/atm/ambassador.c.
This patch converts skb->tail into skb_tail_pointer(skb).

Found by inspection. Compile tested only.

Cc: Simon Horman <horms@verge.net.au>
Cc: Chas Williams <chas@cmf.nrl.navy.mil>
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Reviewed-by: Simon Horman <horms@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/atm/ambassador.c

@@ -1403,7 +1403,7 @@ static void amb_free_rx_skb (struct atm_vcc * atm_vcc, struct sk_buff * skb) {
   rx.host_address = cpu_to_be32 (virt_to_bus (skb->data));
   
   skb->data = skb->head;
-  skb->tail = skb->head;
+  skb_reset_tail_pointer(skb);
   skb->len = 0;
   
   if (!rx_give (dev, &rx, pool)) {