common/dmthin: make this work with external log devices

common/dmthin: make this work with external log devices

Provide a mkfs helper to format the dm thin device when external devices
are in use, and fix the dmthin mount helper to support them.  This fixes
regressions in generic/347 and generic/500 when external logs are in
use.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>