Add state:"nosave" annotation to nvproxy.objsMu.

With this, checkpoint command succeeds on nvproxy containers.

Updates #9363.

PiperOrigin-RevId: 565786118

pkg/sentry/devices/nvproxy/nvproxy.go

@@ -92,7 +92,7 @@ func CreateIndexDevtmpfsFile(ctx context.Context, dev *devtmpfs.Accessor, index
 
 // +stateify savable
 type nvproxy struct {
-	objsMu   objsMutex
+	objsMu   objsMutex `state:"nosave"`
 	objsLive map[nvgpu.Handle]*object
 }