zloop: Add a max iterations option, use default run/pass times

It is useful to have control over the number of iterations of zloop so
we can easily produce "x core dumps found *in y iterations*" metrics.

Using random values for run/pass times doesn't improve coverage in a
meaningful way.

Randomizing run time could be seen as a compromise between running a
greater variety of shorter tests versus a smaller variety of longer
tests within a fixed time span.  However, it is not desirable when
running a fixed number of iterations.

Pass time already incorporates randomness within ztest.

Either parameter can be passed to ztest explicitly if the defaults are
not satisfactory.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #12411

.github/workflows/zloop.yml

@@ -45,7 +45,7 @@ jobs:
       run: |
         sudo mkdir -p $TEST_DIR
         # run for 20 minutes to have a total runner time of 30 minutes
-        sudo /usr/share/zfs/zloop.sh -t 1200 -l -m1
+        sudo /usr/share/zfs/zloop.sh -t 1200 -l -m1 -- -T 120 -P 60
     - name: Prepare artifacts
       if: failure()
       run: |