xen/gntdev: constify mmu_notifier_ops structures

This mmu_notifier_ops structure is never modified, so declare it as
const, like the other mmu_notifier_ops structures.

Done with the help of Coccinelle.
Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

drivers/xen/gntdev.c

@@ -518,7 +518,7 @@ static void mn_release(struct mmu_notifier *mn,
 	mutex_unlock(&priv->lock);
 }
 
-static struct mmu_notifier_ops gntdev_mmu_ops = {
+static const struct mmu_notifier_ops gntdev_mmu_ops = {
 	.release                = mn_release,
 	.invalidate_page        = mn_invl_page,
 	.invalidate_range_start = mn_invl_range_start,