mptcp: count CLOSE-WAIT sockets for MPTCP_MIB_CURRESTAB

Like previous patch does in TCP, we need to adhere to RFC 1213:

  "tcpCurrEstab OBJECT-TYPE
   ...
   The number of TCP connections for which the current state
   is either ESTABLISHED or CLOSE- WAIT."

So let's consider CLOSE-WAIT sockets.

The logic of counting
When we increment the counter?
a) Only if we change the state to ESTABLISHED.

When we decrement the counter?
a) if the socket leaves ESTABLISHED and will never go into CLOSE-WAIT,
say, on the client side, changing from ESTABLISHED to FIN-WAIT-1.
b) if the socket leaves CLOSE-WAIT, say, on the server side, changing
from CLOSE-WAIT to LAST-ACK.

Fixes: d9cd27b ("mptcp: add CurrEstab MIB counter support")
Signed-off-by: Jason Xing <kernelxing@tencent.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/protocol.c

@@ -2916,9 +2916,14 @@ void mptcp_set_state(struct sock *sk, int state)
 		if (oldstate != TCP_ESTABLISHED)
 			MPTCP_INC_STATS(sock_net(sk), MPTCP_MIB_CURRESTAB);
 		break;
-
+	case TCP_CLOSE_WAIT:
+		/* Unlike TCP, MPTCP sk would not have the TCP_SYN_RECV state:
+		 * MPTCP "accepted" sockets will be created later on. So no
+		 * transition from TCP_SYN_RECV to TCP_CLOSE_WAIT.
+		 */
+		break;
 	default:
-		if (oldstate == TCP_ESTABLISHED)
+		if (oldstate == TCP_ESTABLISHED || oldstate == TCP_CLOSE_WAIT)
 			MPTCP_DEC_STATS(sock_net(sk), MPTCP_MIB_CURRESTAB);
 	}