Increase default zloop.sh vdev size

Increase default zloop.sh vdev size

The default 128M vdev size used by zloop.sh isn't always large
enough and can result in ENOSPC failures which suspend the pool.
Increase the default size to 512M and provide a -s option which
can be used to specify an alternate size.

This does increase the free space requirements to run zloop.sh.
However, since the vdevs are sparse 4x the space is not required.

Reviewed-by: Don Brady <don.brady@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6758