RFR: 8313791: Fix just zPage.inline.hpp and xPage.inline.hpp

[Coleen Phillimore]

Please review this trivial change to fix -Wconversion warnings for these two files, which are included almost everywhere so generate lots of warnings.
Tested with tier1 on Oracle supported platforms  linux/macos/windows x86/aarch64.

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Commit messages:
 - 8313791: Fix just zPage.inline.hpp and xPage.inline.hpp

Changes: https://git.openjdk.org/jdk/pull/15166/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15166&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313791
  Stats: 5 lines in 2 files changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/15166.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15166/head:pull/15166

PR: https://git.openjdk.org/jdk/pull/15166