ZTS: Fix add-o_ashift.ksh

Use option '-o' after action for compatibility

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Igor Kozhukhov <igor@dilos.org>
Closes #10426