RFR: 8316337: (bf) Concurrency issue in DirectByteBuffer.Deallocator

[Per Minborg]

This PR proposes to make fields of implementations of the interface `DirectByteBuffer.Deallocator` `final` ensuring safe publication and immutability.

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Commit messages:
 - Make fields final

Changes: https://git.openjdk.org/jdk/pull/15784/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15784&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316337
  Stats: 16 lines in 2 files changed: 4 ins; 2 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/15784.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15784/head:pull/15784

PR: https://git.openjdk.org/jdk/pull/15784