ZTS: zpool_trim_start_and_cancel_pos.ksh

Several of the TRIM tests were based of the initialize tests and
then adapted for TRIM. The zpool_trim_start_and_cancel_pos.ksh
test was intended to be one such test but it was overlooked and
actually never adapted. Update it accordingly.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #11649