zdb: zero-pad checksum output follow up

Apply zero padding for checksums consistently. The SNPRINTF_BLKPTR
macro was not updated in commit ac7648179c8 which results in the
cli_root/zdb/zdb_checksum.ksh test case reliably failing.

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Akash B <akash-b@hpe.com>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #14497