xfs_scrub_all: walk the lsblk device/fs hierarchy correctly

xfs_scrub_all: walk the lsblk device/fs hierarchy correctly

Back when I was designing xfs_scrub_all, I naïvely assumed that the
emitted output would always list physical storage before the virtual
devices stacked atop it.  However, this is not actually true when one
omits the "NAME" column, which is crucial to forcing the output (json or
otherwise) to capture the block device hierarchy.  If the assumption is
violated, the program crashes with a python exception.

To fix this, force the hierarchal json output and restructure the
discovery routines to walk the json object that we receive, from the top
(physical devices) downwards to wherever there are live xfs filesystems.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>