This script was added to provide a simple way to check that zpool
layers correctly on all the standard linux block device types.
It's still a little fragile if there a hiccup in say the md or
lvm tool chain but aside from that it works well.
The 'make check' target now also calls this script in a safe mode
which only operates on files and loopback devices. To check other
block devices types is must be explicitly run by hand because it
will overwrite various block devices.
Brian Behlendorf