The WAL-writing piece was forgetting to set the pages-per-range value.
Also, fix the declared type of struct member heapBlk, which I mistakenly
set as OffsetNumber rather than BlockNumber.

Problem was introduced by commit c655899b (April 1st).  Any system
that tries to replay the new WAL record written before this fix is
likely to die on replay and require pg_resetwal.

Reported by Tom Lane.
Discussion: https://postgr.es/m/20191.1491524824@sss.pgh.pa.us