RFR: 8327097: GenShen: Align PLAB sizes down rather than up

[Y. Srinivas Ramakrishna]

On Mon, 26 Feb 2024 21:27:37 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

> When adjusting LAB sizes, round down rather than rounding up.  Otherwise, we may violate the ShenandoahHumongousThreshold bound.

Changes requested by ysr (Committer).

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PR Review: https://git.openjdk.org/shenandoah/pull/401#pullrequestreview-1923822421