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/privcmd.c b/drivers/xen/privcmd.c
index df2e6f7..702040f 100644
--- a/drivers/xen/privcmd.c
+++ b/drivers/xen/privcmd.c
@@ -582,7 +582,7 @@
 static void privcmd_close(struct vm_area_struct *vma)
 {
 	struct page **pages = vma->vm_private_data;
-	int numpgs = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
+	int numpgs = vma_pages(vma);
 	int numgfns = (vma->vm_end - vma->vm_start) >> XEN_PAGE_SHIFT;
 	int rc;