libata: fix dma_unmap_sg misuse

libata passes the returned value of dma_map_sg() to
dma_unmap_sg(),which is the misuse of dma_unmap_sg().

DMA-mapping.txt says:

To unmap a scatterlist, just call:

	pci_unmap_sg(pdev, sglist, nents, direction);

Again, make sure DMA activity has already finished.

PLEASE NOTE:  The 'nents' argument to the pci_unmap_sg call must be
              the _same_ one you passed into the pci_map_sg call,
	      it should _NOT_ be the 'count' value _returned_ from the
              pci_map_sg call.
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

drivers/ata/libata-core.c

@@ -4612,7 +4612,7 @@ void ata_sg_clean(struct ata_queued_cmd *qc)
 	VPRINTK("unmapping %u sg elements\n", qc->n_elem);
 
 	if (qc->n_elem)
-		dma_unmap_sg(ap->dev, sg, qc->n_elem, dir);
+		dma_unmap_sg(ap->dev, sg, qc->orig_n_elem, dir);
 
 	qc->flags &= ~ATA_QCFLAG_DMAMAP;
 	qc->sg = NULL;
@@ -4727,7 +4727,7 @@ static int ata_sg_setup(struct ata_queued_cmd *qc)
 		return -1;
 
 	DPRINTK("%d sg elements mapped\n", n_elem);
-
+	qc->orig_n_elem = qc->n_elem;
 	qc->n_elem = n_elem;
 	qc->flags |= ATA_QCFLAG_DMAMAP;
 

include/linux/libata.h

@@ -530,6 +530,7 @@ struct ata_queued_cmd {
 	unsigned long		flags;		/* ATA_QCFLAG_xxx */
 	unsigned int		tag;
 	unsigned int		n_elem;
+	unsigned int		orig_n_elem;
 
 	int			dma_dir;