dm-thinp demo test

dm-thinp demo test

Fairly trivial test to use the dm-thin infrastructure.

Right now it exhausts space in queue-on-error mode,
adds more space, does a bit more IO, then unmounts &
checks the fs.

Not sure if that's valid to test, but it works here and
demonstrates the common/dmthin helpers.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>