RFR: 8327097: GenShen: Align PLAB sizes down rather than up [v6]

[Kelvin Nilsen]

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

Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:

  Add comments to clarify behavior of retire_plab

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/401/files
  - new: https://git.openjdk.org/shenandoah/pull/401/files/45695f04..66609d9b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=401&range=05
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=401&range=04-05

  Stats: 5 lines in 1 file changed: 3 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/401.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/401/head:pull/401

PR: https://git.openjdk.org/shenandoah/pull/401