RFR: 8324173: GenShen: Fix error that could cause young gcs to fail when old marking is running

[William Kemper]

Accidentally deleted a line of code in recent PR#382.

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Commit messages:
 - Set requested generation when preempting old mark

Changes: https://git.openjdk.org/shenandoah/pull/385/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=385&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324173
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/385.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/385/head:pull/385

PR: https://git.openjdk.org/shenandoah/pull/385