[jdk17u] RFR: 8275426: PretouchTask num_chunks calculation can overflow

[Aleksey Shipilev]

Unclean backport to fix the overflow in pretouch code. There is a reproducer for x86_32.

The backport does not apply cleanly, because `pretouch_gang` was renamed to `pretouch_workers` later.
@kimbarrett, please ack this version.

Additional testing:
 - [x] Linux x86_32 reproducer
 - [ ] Linux x86_64 fastdebug `tier1`
 - [ ] Linux x86_32 fastdebug `tier1`

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Commit messages:
 - Backport 60cb27dcda475a66e329359ac1ee3ffcde95c657

Changes: https://git.openjdk.java.net/jdk17u/pull/286/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17u&pr=286&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275426
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk17u/pull/286.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17u pull/286/head:pull/286

PR: https://git.openjdk.java.net/jdk17u/pull/286