RFR: 8303749: Serial: Use more strict card table API

Tue Mar 7 13:38:26 UTC 2023

Simple refactoring to skip an unnecessary guard check.

Test: tier1-3

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Commit messages:
 - s1-clear-memregion

Changes: https://git.openjdk.org/jdk/pull/12902/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12902&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303749
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/12902.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12902/head:pull/12902

PR: https://git.openjdk.org/jdk/pull/12902