ztest: reduce runtile of zloop.sh in CI

The zloop.sh script is primarily designed to randomly stress
the DMU and SPA layers. This can result in some unrealistic
(or even impossible) scenarios being tested which then fail.

Since the longer we run zloop.sh the more likely this is to occur
this commit reduces the runtime. The intention being that normally
this will result in a clean CI run unless the PR does introduce
serious breaking change.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #13453