clientconn: go idle if conn closed after preface received

The select branch will be selected randomly, if there are several ready
branches. If the preface has been received and then connection is
closed, the `createTransport` might hit the `connClosed.Done()` branch.
Ideally, the subConn should go idle and reconnect.

Fixes: #5688

Signed-off-by: Wei Fu <fuweid89@gmail.com>

clientconn.go

@@ -1325,9 +1325,15 @@ func (ac *addrConn) createTransport(addr resolver.Address, copts transport.Conne
 		return nil
 	case <-connClosed.Done():
 		// The transport has already closed.  If we received the preface, too,
-		// this is not an error.
+		// this is not an error and go idle.
 		select {
 		case <-prefaceReceived.Done():
+			ac.mu.Lock()
+			defer ac.mu.Unlock()
+
+			if ac.state != connectivity.Shutdown {
+				ac.updateConnectivityState(connectivity.Idle, nil)
+			}
 			return nil
 		default:
 			return errors.New("connection closed before server preface received")