RFR: 8324972: (bf) Make DirectByteBuffer.Deallocator idempotent [v4]

[Per Minborg]

> This PR proposes to make deallocators and unmappers for memory regions idempotent. This is to prevent (likely very rare) duplicate invocations. 
> 
> There are no unit tests but it should be noted that the idempotent behavior (now correct) is similar to the  intended behavior before cf74b8c2a32f33019a13ce80b6667da502cc6722 but where idempotency was not guaranteed in a multi-threaded environment.
> 
> Passes tier1, 2, and 3 tests.

Per Minborg has updated the pull request incrementally with two additional commits since the last revision:

 - Use existing UNSAFE static variable
 - Remove unused import

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/17647/files
  - new: https://git.openjdk.org/jdk/pull/17647/files/21fcd8cf..27b8186a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17647&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17647&range=02-03

  Stats: 4 lines in 2 files changed: 0 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/17647.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17647/head:pull/17647

PR: https://git.openjdk.org/jdk/pull/17647