[SCSI] scsi_debug: disable clustering

scsi_debug does at several places:

for_each_sg(sdb->table.sgl, sg, sdb->table.nents, k) {
	kaddr = (unsigned char *)
		kmap_atomic(sg_page(sg), KM_USER0);

We cannot do something like that with the clustering enabled (or we
can use scsi_kmap_atomic_sg).

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Douglas Gilbert <dougg@torque.net>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>