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>