sparc/iommu: use sbus_iommu_unmap_page in sbus_iommu_unmap_sg

Use the page-level helper instead of duplicating the logic, while also
fixing the incorrect handling of larger than page sized offsets in
the sg variant.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>