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 <[e-mail]>
Cc: Chas Williams <[e-mail]>
Signed-off-by: Isaku Yamahata <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/atm/ambassador.c

···
   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)) {