scsifront: fix address arithmetic

Incrementing the result of page_to_phys() is not the same as invoking
it on the incremented page pointer.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/drivers/xen/scsifront/scsifront.c
+++ b/drivers/xen/scsifront/scsifront.c
@@ -247,7 +247,6 @@ static int map_data_for_request(struct v
 	int err, i, ref, ref_cnt = 0;
 	int write = (sc->sc_data_direction == DMA_TO_DEVICE);
 	int nr_pages, off, len, bytes;
-	unsigned long buffer_pfn;
 	unsigned int data_len = 0;
 
 	if (sc->sc_data_direction == DMA_NONE || !sc->request_bufflen)
@@ -276,8 +275,6 @@ static int map_data_for_request(struct v
 			len = sg[i].length;
 			data_len += len;
 
-			buffer_pfn = page_to_phys(page) >> PAGE_SHIFT;
-
 			while (len > 0) {
 				bytes = min_t(unsigned int, len, PAGE_SIZE - off);
 				
@@ -285,14 +282,14 @@ static int map_data_for_request(struct v
 				BUG_ON(ref == -ENOSPC);
 
 				gnttab_grant_foreign_access_ref(ref, info->dev->otherend_id,
-					buffer_pfn, write);
+					page_to_phys(page) >> PAGE_SHIFT, write);
 
 				info->shadow[id].gref[ref_cnt]  = ref;
 				ring_req->seg[ref_cnt].gref     = ref;
 				ring_req->seg[ref_cnt].offset   = (uint16_t)off;
 				ring_req->seg[ref_cnt].length   = (uint16_t)bytes;
 
-				buffer_pfn++;
+				page++;
 				len -= bytes;
 				off = 0;
 				ref_cnt++;
@@ -312,8 +309,6 @@ static int map_data_for_request(struct v
 			goto big_to_sg;
 		}
 
-		buffer_pfn = page_to_phys(page) >> PAGE_SHIFT;
-
 		off = offset_in_page((unsigned long)sc->request_buffer);
 		for (i = 0; i < nr_pages; i++) {
 			bytes = PAGE_SIZE - off;
@@ -325,14 +320,14 @@ static int map_data_for_request(struct v
 			BUG_ON(ref == -ENOSPC);
 
 			gnttab_grant_foreign_access_ref(ref, info->dev->otherend_id,
-				buffer_pfn, write);
+				page_to_phys(page) >> PAGE_SHIFT, write);
 
 			info->shadow[id].gref[i] = ref;
 			ring_req->seg[i].gref     = ref;
 			ring_req->seg[i].offset   = (uint16_t)off;
 			ring_req->seg[i].length   = (uint16_t)bytes;
 
-			buffer_pfn++;
+			page++;
 			len -= bytes;
 			off = 0;
 			ref_cnt++;