RFR: 8296158: Refactor the verification of CDS region checksum

[Ioi Lam]

Consolidate the two old functions

- `FileMapInfo::region_crc_check(char* buf, size_t size, int expected_crc)`
- `FileMapInfo::verify_region_checksum(int i)`

Into a single function `FileMapRegion::check_region_crc()`.

Also performed some minor renaming and removed a useless API.

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Commit messages:
 - 8296158: Refactor the verification of CDS region checksum

Changes: https://git.openjdk.org/jdk/pull/12424/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12424&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296158
  Stats: 72 lines in 3 files changed: 30 ins; 34 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/12424.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12424/head:pull/12424

PR: https://git.openjdk.org/jdk/pull/12424