When a block is freed, it can be immediately reused if it is from
the current transaction.  But, an extra check is required to make sure
the block had not been written yet.  If it were reused after being written,
the transid in the block header might match the transid of the
next time the block was allocated.

The parent node records the transaction ID of the block it is pointing to,
and this is used as part of validating the block on reads.  So, there
can only be one version of a block per transaction.
Signed-off-by: Chris Mason <chris.mason@oracle.com>