tcp: zero ca_priv area when switching cc algorithms

We need to zero out the private data area when application switches
connection to different algorithm (TCP_CONGESTION setsockopt).

When congestion ops get assigned at connect time everything is already
zeroed because sk_alloc uses GFP_ZERO flag.  But in the setsockopt case
this contains whatever previous cc placed there.
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_cong.c

@@ -200,8 +200,10 @@ static void tcp_reinit_congestion_control(struct sock *sk,
 	icsk->icsk_ca_ops = ca;
 	icsk->icsk_ca_setsockopt = 1;
 
-	if (sk->sk_state != TCP_CLOSE)
+	if (sk->sk_state != TCP_CLOSE) {
+		memset(icsk->icsk_ca_priv, 0, sizeof(icsk->icsk_ca_priv));
 		tcp_init_congestion_control(sk);
+	}
 }
 
 /* Manage refcounts on socket close. */