tcp: prepare CC get_info() access from getsockopt()

We would like that optional info provided by Congestion Control
modules using netlink can also be read using getsockopt()

This patch changes get_info() to put this information in a buffer,
instead of skb, like tcp_get_info(), so that following patch
can reuse this common infrastructure.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/inet_diag.c b/net/ipv4/inet_diag.c
index bb77ebd..4d32262 100644
--- a/net/ipv4/inet_diag.c
+++ b/net/ipv4/inet_diag.c
@@ -224,14 +224,16 @@
 	handler->idiag_get_info(sk, r, info);
 
 	if (sk->sk_state < TCP_TIME_WAIT) {
-		int err = 0;
+		union tcp_cc_info info;
+		size_t sz = 0;
+		int attr;
 
 		rcu_read_lock();
 		ca_ops = READ_ONCE(icsk->icsk_ca_ops);
 		if (ca_ops && ca_ops->get_info)
-			err = ca_ops->get_info(sk, ext, skb);
+			sz = ca_ops->get_info(sk, ext, &attr, &info);
 		rcu_read_unlock();
-		if (err < 0)
+		if (sz && nla_put(skb, attr, sz, &info) < 0)
 			goto errout;
 	}