Unbreak the scan status ABI

When d4a72f23 was merged, pss_pass_issued was incorrectly
added to the middle of the pool_scan_stat_t structure
instead of the end. This patch simply corrects this issue.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #6909