Input: xen-kbdfront - free grant table entry in xenkbd_disconnect_backend

xenkbd_disconnect_backend doesn't free grant table entry.  This bug affects
live migration.

xenkbd_disconnect_backend uses gnttab_end_foreign_access_ref to handle
grant table entry which doesn't really free an entry.

Thus every time we do xenkbd_resume, grant table entry increses by one. As
an grant table entry occupies 8 bytes, an grant table page has at most 512
entries. Every 512 times we do xenkdb_resume, grant table pages increses by
one.

After around 3500 times of live migration, grant table pages will increase
by 7, causing too many pages to populate and hitting max_pages limit when
assigning pages.Thus assign_pages will fail, so will live migration.
Signed-off-by: Chang Huaixin <huaixin.chx@alibaba-inc.com>
Acked-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/misc/xen-kbdfront.c

@@ -285,7 +285,7 @@ static int xenkbd_connect_backend(struct xenbus_device *dev,
  error_evtchan:
 	xenbus_free_evtchn(dev, evtchn);
  error_grant:
-	gnttab_end_foreign_access_ref(info->gref, 0);
+	gnttab_end_foreign_access(info->gref, 0, 0UL);
 	info->gref = -1;
 	return ret;
 }
@@ -296,7 +296,7 @@ static void xenkbd_disconnect_backend(struct xenkbd_info *info)
 		unbind_from_irqhandler(info->irq, info);
 	info->irq = -1;
 	if (info->gref >= 0)
-		gnttab_end_foreign_access_ref(info->gref, 0);
+		gnttab_end_foreign_access(info->gref, 0, 0UL);
 	info->gref = -1;
 }