generic/474: shift target directory to a sub-dir of SCRATCH_MNT

generic/474: shift target directory to a sub-dir of SCRATCH_MNT

If testing on large fs (--large-fs option), there's a huge size
.use_space file in $SCRATCH_MNT, then `fssum $SCRATCH_MNT` trys to
read whole huge file. That's wasting time, so change the target path
to a sub-dir of $SCRATCH_MNT.

Signed-off-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>