syncookies: Make sure ECN is disabled

ecn_ok is not initialized when a connection is established by cookies.
The cookie syn-ack never sets ECN, so ecn_ok must be set to 0.

Spotted using ns-3/network simulation cradle simulator and valgrind.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>