RFR: 8314161: G1: Fix -Wconversion warnings in G1CardSetConfiguration::_bitmap_hash_mask

[Albert Mingkun Yang]

Simple type change and updating to use more common mask-initialization.

Test: hotspot_gc

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Commit messages:
 - g1-mask

Changes: https://git.openjdk.org/jdk/pull/15247/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15247&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314161
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/15247.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15247/head:pull/15247

PR: https://git.openjdk.org/jdk/pull/15247