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 <[email protected]>
Reported-By: Erik Nordström <[email protected]>
Reviewed-by: Erik Nordström <[email protected]>
Discussion: 
https://www.postgresql.org/message-id/cacaa4vj%[email protected]

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/ee937f0409d5c75855861bf31f88eeb77623b411

Modified Files
--------------
src/backend/storage/smgr/bulk_write.c | 19 +++++++++++--------
1 file changed, 11 insertions(+), 8 deletions(-)

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