pg_freespacemap: Fix declaration of pg_freespace(regclass)

pg_freespacemap: Fix declaration of pg_freespace(regclass)

This function called generate_series() without enforcing its input
argument types, making possible for an attacker to catch this call, by
defining for example a generate_series(int,bigint).

The internals of pg_freespace(regclass) are changed to force the use of
bigint for the inputs of generate_series().  A more consistent style is
applied for all its hardcoded values, while on it.

Issue introduced in 3f323eba89fb.

Reported-by: Noah Misch
Reviewed-by: Noah Misch
Discussion: https://postgr.es/m/20250106190428.ec.nmisch@google.com