Add extra disconnect safety (minio#19022)

Fix reported races that are actually synchronized by network calls.

But this should add some extra safety for untimely disconnects.

Race reported:

```
WARNING: DATA RACE
Read at 0x00c00171c9c0 by goroutine 214:
  github.com/minio/minio/internal/grid.(*muxClient).addResponse()
      e:/gopath/src/github.com/minio/minio/internal/grid/muxclient.go:519 +0x111
  github.com/minio/minio/internal/grid.(*muxClient).error()
      e:/gopath/src/github.com/minio/minio/internal/grid/muxclient.go:470 +0x21d
  github.com/minio/minio/internal/grid.(*Connection).handleDisconnectClientMux()
      e:/gopath/src/github.com/minio/minio/internal/grid/connection.go:1391 +0x15b
  github.com/minio/minio/internal/grid.(*Connection).handleMsg()
      e:/gopath/src/github.com/minio/minio/internal/grid/connection.go:1190 +0x1ab
  github.com/minio/minio/internal/grid.(*Connection).handleMessages.func1()
      e:/gopath/src/github.com/minio/minio/internal/grid/connection.go:981 +0x610

Previous write at 0x00c00171c9c0 by goroutine 1081:
  github.com/minio/minio/internal/grid.(*muxClient).roundtrip()
      e:/gopath/src/github.com/minio/minio/internal/grid/muxclient.go:94 +0x324
  github.com/minio/minio/internal/grid.(*muxClient).traceRoundtrip()
      e:/gopath/src/github.com/minio/minio/internal/grid/trace.go:74 +0x10e4
  github.com/minio/minio/internal/grid.(*Subroute).Request()
      e:/gopath/src/github.com/minio/minio/internal/grid/connection.go:366 +0x230
  github.com/minio/minio/internal/grid.(*SingleHandler[go.shape.*github.com/minio/minio/cmd.DiskInfoOptions,go.shape.*github.com/minio/minio/cmd.DiskInfo]).Call()
      e:/gopath/src/github.com/minio/minio/internal/grid/handlers.go:554 +0x3fd
  github.com/minio/minio/cmd.(*storageRESTClient).DiskInfo()
      e:/gopath/src/github.com/minio/minio/cmd/storage-rest-client.go:314 +0x270
  github.com/minio/minio/cmd.erasureObjects.getOnlineDisksWithHealingAndInfo.func1()
      e:/gopath/src/github.com/minio/minio/cmd/erasure.go:293 +0x171
```

This read will always happen after the write, since there is a network call in between.

However a disconnect could come in while we are setting up the call, so we protect against that with extra checks.

Author: klauspost

internal/grid/debugmsg_string.go

@@ -91,7 +91,13 @@ func (m *muxClient) roundtrip(h HandlerID, req []byte) ([]byte, error) {
 		msg.Flags |= FlagSubroute
 	}
 	ch := make(chan Response, 1)
+	m.respMu.Lock()
+	if m.closed {
+		m.respMu.Unlock()
+		return nil, ErrDisconnected
+	}
 	m.respWait = ch
+	m.respMu.Unlock()
 	ctx := m.ctx
 
 	// Add deadline if none.
@@ -101,8 +107,8 @@ func (m *muxClient) roundtrip(h HandlerID, req []byte) ([]byte, error) {
 		ctx, cancel = context.WithTimeout(ctx, defaultSingleRequestTimeout)
 		defer cancel()
 	}
-	// Send... (no need for lock yet)
-	if err := m.sendLocked(msg); err != nil {
+	// Send request
+	if err := m.send(msg); err != nil {
 		return nil, err
 	}
 	if debugReqs {
@@ -215,7 +221,13 @@ func (m *muxClient) RequestStream(h HandlerID, payload []byte, requests chan []b
 		return nil, errors.New("RequestStream: responses channel is nil")
 	}
 	m.init = true
+	m.respMu.Lock()
+	if m.closed {
+		m.respMu.Unlock()
+		return nil, ErrDisconnected
+	}
 	m.respWait = responses // Route directly to output.
+	m.respMu.Unlock()
 
 	// Try to grab an initial block.
 	m.singleResp = false