Make sure no callback is running before removing the module.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Kent Overstreet <kmo@daterainc.com>
---
 drivers/block/umem.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: tip/drivers/block/umem.c
===================================================================
--- tip.orig/drivers/block/umem.c
+++ tip/drivers/block/umem.c
@@ -746,7 +746,7 @@ static void init_battery_timer(void)
 
 static void del_battery_timer(void)
 {
-	del_timer(&battery_timer);
+	del_timer_sync(&battery_timer);
 }
 
 /*