RFR: 8296158: Refactor the verification of CDS region checksum [v2]

[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.

Ioi Lam has updated the pull request incrementally with one additional commit since the last revision:

  @matias9927 suggestions

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/12424/files
  - new: https://git.openjdk.org/jdk/pull/12424/files/3ad8d838..10dcd0b1

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12424&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12424&range=00-01

  Stats: 7 lines in 1 file changed: 0 ins; 3 del; 4 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