tcp: Document use of undefined variable.

Both tcp_timewait_state_process and tcp_check_req use the same basic
construct of

	struct tcp_options received tmp_opt;
	tmp_opt.saw_tstamp = 0;

then call

	tcp_parse_options

However if they are fed a frame containing a TCP_SACK then tbe code
behaviour is undefined because opt_rx->sack_ok is undefined data.

This ought to be documented if it is intentional.
Signed-off-by: NAlan Cox <alan@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_minisocks.c

@@ -85,6 +85,8 @@ static bool tcp_in_window(u32 seq, u32 end_seq, u32 s_win, u32 e_win)
  * spinlock it. I do not want! Well, probability of misbehaviour
  * is ridiculously low and, seems, we could use some mb() tricks
  * to avoid misread sequence numbers, states etc.  --ANK
+ *
+ * We don't need to initialize tmp_out.sack_ok as we don't use the results
  */
 enum tcp_tw_status
 tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
@@ -522,6 +524,8 @@ EXPORT_SYMBOL(tcp_create_openreq_child);
  *
  * XXX (TFO) - The current impl contains a special check for ack
  * validation and inside tcp_v4_reqsk_send_ack(). Can we do better?
+ *
+ * We don't need to initialize tmp_opt.sack_ok as we don't use the results
  */
 
 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,