Fix bug introduced by recent SSI patch to merge ROLLED_BACK and

Fix bug introduced by recent SSI patch to merge ROLLED_BACK and
MARKED_FOR_DEATH flags into one. We still need the ROLLED_BACK flag to
mark transactions that are in the process of being rolled back. To be
precise, ROLLED_BACK now means that a transaction has already been
discounted from the count of transactions with the oldest xmin, but not
yet removed from the list of active transactions.

Dan Ports