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

[Lin Zang]

The root cause for the fail is that there must be a flush for `BufferedOutputStream` before getting the `pos` of the file, otherwise the data length is wrongly calculated.

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Commit messages:
 - 8275021: Test serviceability/sa/TestJmapCore.java fails

Changes: https://git.openjdk.java.net/jdk/pull/5890/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5890&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275021
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5890.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5890/head:pull/5890

PR: https://git.openjdk.java.net/jdk/pull/5890