xen: use vma_pages().

Replace explicit computation of vma page count by a call to
vma_pages().

Signed-off-by: Muhammad Falak R Wani <falakreyaz@gmail.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index 6793957..bb95212 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -982,7 +982,7 @@
 {
 	struct gntdev_priv *priv = flip->private_data;
 	int index = vma->vm_pgoff;
-	int count = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
+	int count = vma_pages(vma);
 	struct grant_map *map;
 	int i, err = -EINVAL;