Remove unnecessary (char *) casts [checksum]

Remove unnecessary (char *) casts [checksum]

Remove some (char *) casts related to uses of the pg_checksum_page()
function.  These casts are useless, because everything involved
already has the right type.  Moreover, these casts actually silently
discarded a const qualifier.  The declaration of a higher-level
function needs to be adjusted to fix that.

Reviewed-by: Dagfinn Ilmari Mannsåker 
Discussion: https://www.postgresql.org/message-id/flat/fd1fcedb-3492-4fc8-9e3e-74b97f2db6c7%40eisentraut.org