RFR: 8316337: (bf) Concurrency issue in DirectByteBuffer.Deallocator [v2]

[Per Minborg]

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

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Make the unmapper of MappedByteBuffer safe

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15784/files
  - new: https://git.openjdk.org/jdk/pull/15784/files/83a30d3e..e8ed778b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15784&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15784&range=00-01

  Stats: 33 lines in 1 file changed: 11 ins; 0 del; 22 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