RFR: 8254972: Fix pretouch chunk calculations

[Thomas Schatzl]

Hi all,

  can I have reviews for this follow-up to JDK-8252221 where Kim noticed that the code in the pretouch task has some minor issues?

In particular, some code could be simplified and due to some inconsistencies, and used less than ideal number of workers. There is also a fix to avoid an overflow if you want to pretouch >= 16TB of heap with 4k pages (order of casts in calculation of ``num_workers``).

Testing: tier1

Thanks,
  Thomas

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Commit messages:
 - Initial import

Changes: https://git.openjdk.java.net/jdk/pull/884/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=884&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254972
  Stats: 29 lines in 2 files changed: 13 ins; 7 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/884.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/884/head:pull/884

PR: https://git.openjdk.java.net/jdk/pull/884