xen/grant-table: add helpers for allocating pages

Add gnttab_alloc_pages() and gnttab_free_pages() to allocate/free pages
suitable to for granted maps.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index 6444172..8cc3f06 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -119,7 +119,7 @@
 		return;
 
 	if (map->pages)
-		free_xenballooned_pages(map->count, map->pages);
+		gnttab_free_pages(map->count, map->pages);
 	kfree(map->pages);
 	kfree(map->grants);
 	kfree(map->map_ops);
@@ -152,7 +152,7 @@
 	    NULL == add->pages)
 		goto err;
 
-	if (alloc_xenballooned_pages(count, add->pages, false /* lowmem */))
+	if (gnttab_alloc_pages(count, add->pages))
 		goto err;
 
 	for (i = 0; i < count; i++) {