RFR: 8321971: Improve the user name detection logic in perfMemory get_user_name_slow [v2]

[Jaikiran Pai]

> Can I please get a review of this change which proposes to improve the code in `get_user_name_slow` function, which is used to identify the target JVM owner's user name? This addresses https://bugs.openjdk.org/browse/JDK-8321971.
> 
> As noted in that JBS issue, in its current form, the nested loop ends up iterating over the directory contents of `hsperfdata_xxx` directory and then for each iteration it checks if the name of the entry matches the pid. This iteration shouldn't be needed and instead one could look for a file named `<pid>` within that directory.
> 
> No new test has been added, given the nature of this change. Existing tier1, tier2, tier3 and svc_tools tests pass with this change on Linux, Windows and macosx.

Jaikiran Pai has updated the pull request incrementally with two additional commits since the last revision:

 - David's review comments - reduce if blocks and release the array outside if block
 - David's review comment - punctuation

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17104/files
  - new: https://git.openjdk.org/jdk/pull/17104/files/4d428363..cfd50d79

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17104&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17104&range=00-01

  Stats: 25 lines in 2 files changed: 0 ins; 14 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/17104.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17104/head:pull/17104

PR: https://git.openjdk.org/jdk/pull/17104