RFR: 8366421: ModifiedUtf.utfLen may overflow for giant string

[Guanqiang Han]

Please review this patch.

**Description:**

Currently, ModifiedUtf.utfLen returns a signed int. For very large strings, this may overflow and produce negative values, leading to incorrect behavior in code that relies on the UTF length. This patch changes the return type to long, which fully resolves the issue and allows safe handling of giant strings.

**Test:**

GHA

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Commit messages:
 - Change return type of utfLen to long to prevent overflow

Changes: https://git.openjdk.org/jdk/pull/27285/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27285&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8366421
  Stats: 24 lines in 4 files changed: 5 ins; 3 del; 16 mod
  Patch: https://git.openjdk.org/jdk/pull/27285.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27285/head:pull/27285

PR: https://git.openjdk.org/jdk/pull/27285