ocl: silence warning in case of async cleanup

- OpenCL kernel cleanup processing is asynchronous and can be called even after forced clFinish()
- buffers are released later in asynchronous mode
- silence these false positive cases for asynchronous cleanup

modules/core/src/umatrix.cpp

@@ -80,6 +80,7 @@ UMatData::~UMatData()
     CV_Assert(mapcount == 0);
     data = origdata = 0;
     size = 0;
+    bool isAsyncCleanup = !!(flags & UMatData::ASYNC_CLEANUP);
     flags = 0;
     handle = 0;
     userdata = 0;
@@ -106,7 +107,7 @@ UMatData::~UMatData()
             showWarn = true;
         if (zero_Ref && zero_URef) // oops, we need to free resources
         {
-            showWarn = true;
+            showWarn = !isAsyncCleanup;
             // simulate UMat::deallocate
             u->currAllocator->deallocate(u);
         }