xfs: fix $XFS_DB_PROG usage

xfs: fix $XFS_DB_PROG usage

xfs_db requires us to pass in the log device, if any; this can be
accomplished via _scratch_xfs_db_options (if we're operating on the
scratch device, anyway).  However, many of the tests/xfs/ scripts
pass only $SCRATCH_DEV directly, so they'll fail if we test with an
external log.  Fix that by adding a new _scratch_xfs_db helper.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eryu Guan <eguan@redhat.com>