ZTS: Fix zdb_display_block on FreeBSD

ZTS: Fix zdb_display_block on FreeBSD

Missed this in the review, but wc output on FreeBSD is indented,
so string comparisons mismatch when comparing to an unindented number.

Compare counts as integers instead of strings.

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Paul Zuchowski <pzuchowski@datto.com>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #9980