8275021: Test serviceability/sa/TestJmapCore.java fails with: java.io.IOException: Stack frame 0x4 not found

Reviewed-by: dholmes, dcubed

src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/utilities/HeapHprofBinWriter.java

@@ -615,6 +615,8 @@ public class HeapHprofBinWriter extends AbstractHeapGraphWriter {
         } else {
             out.writeByte((byte)HPROF_HEAP_DUMP);
             out.writeInt(0);
+            // We must flush all data to the file before reading the current file position.
+            out.flush();
             // record the current position in file, it will be use for calculating the size of written data
             currentSegmentStart = fos.getChannel().position();
             // write dummy zero for length