Don't count zero-filled buffers as 'read' in EXPLAIN.

Don't count zero-filled buffers as 'read' in EXPLAIN.

If you extend a relation, it should count as a block written, not
read (we write a zero-filled block).  If you ask for a zero-filled
buffer, it shouldn't be counted as read or written.

Later we might consider counting zero-filled buffers with a separate
counter, if they become more common due to future work.

Author: Thomas Munro
Reviewed-by: Haribabu Kommi, Kyotaro Horiguchi, David Rowley
Discussion: https://postgr.es/m/CAEepm%3D3JytB3KPpvSwXzkY%2Bdwc5zC8P8Lk7Nedkoci81_0E9rA%40mail.gmail.com