RFR: 8276129: PretouchTask should page-align the chunk size

[Kim Barrett]

Please review this change to PretouchTask to page-align the chunk size.
Aligning down is used because it's easy; aligning up would require dealing
with possible overflow or verifying that overflow isn't possible.

Testing:
mach5 tier1

Locally (linux-x64) java -version with pretouch task logging and various
chunk sizes, manually verifying the number of workers was as expected.

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Commit messages:
 - page-align chunk size

Changes: https://git.openjdk.java.net/jdk/pull/6204/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6204&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276129
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6204.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6204/head:pull/6204

PR: https://git.openjdk.java.net/jdk/pull/6204