RFR: 8316417: ObjectMonitorIterator does not return the most recent monitor and is incorrect if no monitors exists [v5]

[Axel Boldt-Christmas]

> ObjectMonitorIterator fails to return the most resent monitor added. It start with returning the `nextOM()` ObjectMonitor from the `_head` ObjectMonitor but fails to ever return the `_head` ObjectMonitor.
> The current implementation can also not handle that the `_head` is nullptr (no monitors in the system) and returns a null ObjectMonitorIterator. Which is interpreted as `monitor list not supported, too old hotspot VM`.
> 
> Changed the iterator to keep return the current monitor (starts with `_head`) and decoupled `_head == nullptr` from the question if ObjectMonitorIterator is supported. 
> 
> Testing:
>   * Passes all `serviceability/sa` tests
>   * Currently running tier 1-3
>   * Currently running GHA

Axel Boldt-Christmas has updated the pull request incrementally with one additional commit since the last revision:

  Removed unnecessary comment

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/15782/files
  - new: https://git.openjdk.org/jdk/pull/15782/files/48a6ba80..faa579a9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15782&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15782&range=03-04

  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15782.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15782/head:pull/15782

PR: https://git.openjdk.org/jdk/pull/15782