[jdk17u-dev] RFR: 8319048: Monitor deflation unlink phase prolongs time to safepoint

[Aleksey Shipilev]

Unclean backport to eliminate another safepoint hiccup caused by monitor deflation. The uncleanliness comes mostly from `NULL` vs `nullptr` differences in affected hunks. I chose to accept the new version that uses `nullptr` consistently, even though some `NULL`-s might have been left alone.

Additional testing:
  - [x] Linux x86_64 server fastdebug, `all` tests pass

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Commit messages:
 - Backport efc392259c64986bbbe880259e95b09058b9076a

Changes: https://git.openjdk.org/jdk17u-dev/pull/2309/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=2309&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319048
  Stats: 240 lines in 4 files changed: 222 ins; 0 del; 18 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/2309.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/2309/head:pull/2309

PR: https://git.openjdk.org/jdk17u-dev/pull/2309