RFR: 8257750: writeBuffer field of java.io.DataOutputStream should be final

[Brian Burkhalter]

Please review this trivial change to make the `writeBuffer` array field final and move it to the beginning of the class where other fields are declared.

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Commit messages:
 - 8257750: writeBuffer field of java.io.DataOutputStream should be final

Changes: https://git.openjdk.java.net/jdk/pull/1627/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1627&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257750
  Stats: 4 lines in 1 file changed: 2 ins; 2 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1627.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1627/head:pull/1627

PR: https://git.openjdk.java.net/jdk/pull/1627