generic/095, generic/247: unmount correct devices

generic/095, generic/247: unmount correct devices

In commit d55123c080cb ("generic/247: filter out expected XFS warnings
for mixed mmap/direct I/O"), _scratch_unmount was removed from
generic/095 and added to generic/247.  But actually generic/095 *should*
be unmounting SCRATCH_DEV; and generic/247 should be unmounting
TEST_DEV, not SCRATCH_DEV, since it doesn't use a scratch device (it was
failing if SCRATCH_DEV was not defined).  Fix it.

[eguan: the original bug was introduced by me not Brian, as I
removed _scratch_unmount from wrong test]

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>