RFR: 8357976: GenShen crash in swap_card_tables: Should be clean

[Y. Srinivas Ramakrishna]

On Tue, 10 Jun 2025 20:14:49 GMT, William Kemper <wkemper at openjdk.org> wrote:

> Degenerated cycles must also clean the `read` card table before swapping it with the `write` card table.

Suggest moving the reset to a more consolidated natural point rather than piecemeal as done in this solution.

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Changes requested by ysr (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/25735#pullrequestreview-2915255444