scsi: pmcraid: use sg helper to iterate over scatterlist

Unlike the legacy I/O path, scsi-mq preallocates a large array to hold
the scatterlist for each request. This static allocation can consume
substantial amounts of memory on modern controllers which support a
large number of concurrently outstanding requests.

To facilitate a switch to a smaller static allocation combined with a
dynamic allocation for requests that need it, we need to make sure all
SCSI drivers handle chained scatterlists correctly.

Convert remaining drivers that directly dereference the scatterlist
array to using the iterator functions.

[mkp: clarified commit message]

Reviewed-by: Christoph Hellwig <[e-mail]>
Reviewed-by: Bart Van Assche <[e-mail]>
Signed-off-by: Ming Lei <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/scsi/pmcraid.c

···
 	int direction
 )
 {
-	struct scatterlist *scatterlist;
+	struct scatterlist *sg;
 	void *kaddr;
 	int bsize_elem;
 	int i;
···
 	/* Determine the actual number of bytes per element */
 	bsize_elem = PAGE_SIZE * (1 << sglist->order);
 
-	scatterlist = sglist->scatterlist;
+	sg = sglist->scatterlist;
 
-	for (i = 0; i < (len / bsize_elem); i++, buffer += bsize_elem) {
-		struct page *page = sg_page(&scatterlist[i]);
+	for (i = 0; i < (len / bsize_elem); i++, sg = sg_next(sg), buffer += bsize_elem) {
+		struct page *page = sg_page(sg);
 
 		kaddr = kmap(page);
 		if (direction == DMA_TO_DEVICE)
···
 			return -EFAULT;
 		}
 
-		scatterlist[i].length = bsize_elem;
+		sg->length = bsize_elem;
 	}
 
 	if (len % bsize_elem) {
-		struct page *page = sg_page(&scatterlist[i]);
+		struct page *page = sg_page(sg);
 
 		kaddr = kmap(page);
 
···
 
 		kunmap(page);
 
-		scatterlist[i].length = len % bsize_elem;
+		sg->length = len % bsize_elem;
 	}
 
 	if (rc) {