xen/xenbus: Don't leak memory when unmapping the ring on HVM backend

The commit ccc9d90a "xenbus_client:
Extend interface to support multi-page ring" removes the call to
free_xenballooned_pages() in xenbus_unmap_ring_vfree_hvm(), leaking a
page for every shared ring.

Only with backends running in HVM domains were affected.
Signed-off-by: Julien Grall <julien.grall@citrix.com>
Cc: <stable@vger.kernel.org>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

drivers/xen/xenbus/xenbus_client.c

@@ -814,8 +814,10 @@ static int xenbus_unmap_ring_vfree_hvm(struct xenbus_device *dev, void *vaddr)
 
 	rv = xenbus_unmap_ring(dev, node->handles, node->nr_handles,
 			       addrs);
-	if (!rv)
+	if (!rv) {
 		vunmap(vaddr);
+		free_xenballooned_pages(node->nr_handles, node->hvm.pages);
+	}
 	else
 		WARN(1, "Leaking %p, size %u page(s)\n", vaddr,
 		     node->nr_handles);