tcp: Fix out of bounds access to tcpm_vals

The recent patch "tcp: Maintain dynamic metrics in local cache." introduced
an out of bounds access due to what appears to be a typo.   I believe this
change should resolve the issue by replacing the access to RTAX_CWND with
TCP_METRIC_CWND.
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_metrics.c

@@ -412,7 +412,7 @@ void tcp_update_metrics(struct sock *sk)
 				       max(tp->snd_cwnd >> 1, tp->snd_ssthresh));
 		if (!tcp_metric_locked(tm, TCP_METRIC_CWND)) {
 			val = tcp_metric_get(tm, TCP_METRIC_CWND);
-			tcp_metric_set(tm, RTAX_CWND, (val + tp->snd_cwnd) >> 1);
+			tcp_metric_set(tm, TCP_METRIC_CWND, (val + tp->snd_cwnd) >> 1);
 		}
 	} else {
 		/* Else slow start did not finish, cwnd is non-sense,