tcp: initialize variable ecn_ok in syncookies path

Using a gcc 4.4.3, warnings are emitted for a possibly uninitialized use
of ecn_ok.

This can happen if cookie_check_timestamp() returns due to not having
seen a timestamp.  Defaulting to ecn off seems like a reasonable thing
to do in this case, so initialized ecn_ok to false.

Signed-off-by: Mike Waychison <mikew@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>