zloop.sh should call ZDB with pool name

zloop.sh should call ZDB with pool name

Commit 54007c79 introduced an error, changing the final
argument to $ZDB from ztest to $ZTEST.  This argument
indicates the pool name, not the script, and so should
not have been changed.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes #10118