Update vdev_is_spacemap_addressable() for new spacemap encoding

Since the new spacemap encoding was ported to ZoL that's no longer
a limitation. This patch updates vdev_is_spacemap_addressable()
that was performing that check.

It also updates the appropriate test to ensure that the same
functionality is tested. The test does so by creating pools that
don't have the new spacemap encoding enabled - just the checkpoint
feature. This patch also reorganizes that same tests in order to
cut in half its memory consumption.

Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Closes #8286