common/dmlogwrites: genericize log writes target device

common/dmlogwrites: genericize log writes target device

The dm-log-writes infrastructure is currently implemented to use
SCRATCH_DEV as a hardcoded data device. In preparation to allow use
of specialized devices in certain circumstances, genericize the code
to allow an arbitrary data device. This requires passing the target
device as a parameter to several helper functions from various
tests. No functional changes.

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