7192128: G1: Extend fix for 6948537 to G1's BOT Summary: G1 does not appear to be immune to the issue described in CR 6948537 and increasing the size of old-generation PLABs appears to increase the liklihood of seeing the issue. Extend the fix for 6948537 to G1's BlockOffsetTable. Reviewed-by: brutisso, jmasa

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