Illumos 5039 - ztest should default to larger device sizes

5039 ztest should default to larger device sizes
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Max Grossman <max.grossman@delphix.com>
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: Saso Kiselkov <skiselkov.ml@gmail.com>
Approved by: Richard Lowe <richlowe@richlowe.net>

References:
  https://www.illumos.org/issues/5039
  illumos/illumos-gate@539eed8

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

cmd/ztest/ztest.c

@@ -175,7 +175,7 @@ static const ztest_shared_opts_t ztest_opts_defaults = {
 	.zo_mirrors = 2,
 	.zo_raidz = 4,
 	.zo_raidz_parity = 1,
-	.zo_vdev_size = SPA_MINDEVSIZE,
+	.zo_vdev_size = SPA_MINDEVSIZE * 2,
 	.zo_datasets = 7,
 	.zo_threads = 23,
 	.zo_passtime = 60,		/* 60 seconds */