xen/blkback: simplify address translations

Cherry-pick and modified from 69d64727c42eecd47fdf82c15a54474d21a4012a
("blkback/blktap2: simplify address translations"):

"There are quite a number of places where e.g. page->va->page
translations happen.

Besides yielding smaller code (source and binary), a second goal is to
make it easier to determine where virtual addresses of pages allocated
through alloc_empty_pages_and_pagevec() are really used (in turn in
order to determine whether using highmem pages would be possible
there)."

The second goal is not the purpose of this patch - it is just to
make it easier to read the code.

linux-2.6-pvops:
 * Stripped drivers/xen/gntdev/*
 * Stripped drivers/xen/netback/*

[v2: Stripped blktap off]
Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Daniel Stodden <daniel.stodden@citrix.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/blkback/blkback.c

@@ -99,9 +99,11 @@ static inline int vaddr_pagenr(pending_req_t *req, int seg)
 	return (req - pending_reqs) * BLKIF_MAX_SEGMENTS_PER_REQUEST + seg;
 }
 
+#define pending_page(req, seg) pending_pages[vaddr_pagenr(req, seg)]
+
 static inline unsigned long vaddr(pending_req_t *req, int seg)
 {
-	unsigned long pfn = page_to_pfn(pending_pages[vaddr_pagenr(req, seg)]);
+	unsigned long pfn = page_to_pfn(pending_page(req, seg));
 	return (unsigned long)pfn_to_kaddr(pfn);
 }
 
@@ -463,8 +465,8 @@ static void dispatch_rw_block_io(blkif_t *blkif,
 		if (ret)
 			continue;
 
-		set_phys_to_machine(__pa(vaddr(
-			pending_req, i)) >> PAGE_SHIFT,
+		set_phys_to_machine(
+			page_to_pfn(pending_page(pending_req, i)),
 			FOREIGN_FRAME(map[i].dev_bus_addr >> PAGE_SHIFT));
 		seg[i].buf  = map[i].dev_bus_addr |
 			(req->seg[i].first_sect << 9);
@@ -495,7 +497,7 @@ static void dispatch_rw_block_io(blkif_t *blkif,
 
 		while ((bio == NULL) ||
 		       (bio_add_page(bio,
-				     virt_to_page(vaddr(pending_req, i)),
+				     pending_page(pending_req, i),
 				     seg[i].nsec << 9,
 				     seg[i].buf & ~PAGE_MASK) == 0)) {
 			if (bio) {