generic/482: use thin volume as data device

generic/482: use thin volume as data device

The dm-log-writes replay mechanism issues discards to provide
zeroing functionality to prevent out-of-order replay issues. These
discards don't always result in zeroing bevavior, however, depending
on the underlying physical device. In turn, this causes test
failures on XFS v5 filesystems that enforce metadata log recovery
ordering if the filesystem ends up with stale data from the future
with respect to the active log at a particular recovery point.

To ensure reliable discard zeroing behavior, use a thinly
provisioned volume as the data device instead of using the scratch
device directly. This slows the test down slightly, but provides
reliable functional behavior at a reduced cost from active snapshot
management or forced zeroing.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>