Dynamically create loop devices

Several of the in-tree regression tests depend on the availability
of loop devices.  If for some reason no loop devices are available
the tests will fail.

Normally this isn't as issue because most Linux distributions create
8 loop devices by default.  This is enough for our purposes.  However,
recent Fedora releases have only been creating a single loop device
and this leads to failures when testing in a pristine VM.  Alternately,
if something else of the system is using the loop devices we may see
failures.

The fix for this is to update the support scripts to dynamically
create loop devices as needed.  The scripts need only create a node
under /dev/ and the loop driver with create the minor.  This behavior
has supported by the loop driver for ages.

Additionally this patch updates cleanup_loop_devices() to cleanup
loop devices which have already had their file store deleted.  This
helps prevent stale loop devices from accumulating on the system due
to test failures.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>