Just had a case with a gdth adapter not working in a 16GB box. Seems to
me that the SCSI bounce decision is completely screwy - it returns the
device dma mask, if it is set only and the box doesn't have an iommu. So
if we can't get the mask, we allow any io. Not so good. I think the
logic should be more as follows:

	if (isa)
		return bounce_above_isa;
	if (has_iommu)
		return bounce_nothing;

	if (we_know_device_dma_mask)
		return mask;
	else
		return default_to_32bit;

Patch corrects scsi_calculate_bounce_limit() to actually work partly
sanely.
Signed-off-by: Jens Axboe <axboe@suse.de>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>