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 <alan@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>