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: Alan Cox <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/tcp_minisocks.c

···
  * 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,
···
  *
  * 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,