Fix data loss when restarting the bulk_write facility

Fix data loss when restarting the bulk_write facility

If a user started a bulk write operation on a fork with existing data
to append data in bulk, the bulk_write machinery would zero out all
previously written pages up to the last page written by the new
bulk_write operation.

This is not an issue for PostgreSQL itself, because we never use the
bulk_write facility on a non-empty fork. But there are use cases where
it makes sense. TimescaleDB extension is known to do that to merge
partitions, for example.

Backpatch to v17, where the bulk_write machinery was introduced.

Author: Matthias van de Meent 
Reported-By: Erik Nordström 
Reviewed-by: Erik Nordström 
Discussion: https://www.postgresql.org/message-id/CACAa4VJ%[email protected]