RFR: 8273505: runtime/cds/appcds/loaderConstraints/DynamicLoaderConstraintsTest.java#default-cl crashed with SIGSEGV in MetaspaceShared::link_shared_classes

[Calvin Cheung]

Please review this small patch for keeping the class loader, which is the holder of the `ClassLoaderData`, alive during class linking within `MetaspaceShared::link_shared_classes()`. Prior to this fix, the `ClassLoaderData::inc_keep_alive()` was used in an attempt to keep the `ClassLoaderData` alive but that was for hidden classes only.

Testing: tiers 1 - 4.

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Commit messages:
 - 8273505: runtime/cds/appcds/loaderConstraints/DynamicLoaderConstraintsTest.java#default-cl crashed with SIGSEGV in MetaspaceShared::link_shared_classes

Changes: https://git.openjdk.java.net/jdk/pull/5566/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5566&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273505
  Stats: 15 lines in 2 files changed: 5 ins; 7 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5566.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5566/head:pull/5566

PR: https://git.openjdk.java.net/jdk/pull/5566