backing-dev: use synchronize_rcu_expedited instead of synchronize_rcu

backing-dev: use synchronize_rcu_expedited instead of synchronize_rcu

synchronize_rcu sleeps several timer ticks. synchronize_rcu_expedited is
much faster.

With 100Hz timer frequency, when we remove 10000 block devices with
"dmsetup remove_all" command, it takes 27 minutes. With this patch,
removing 10000 block devices takes only 15 seconds.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

mm/backing-dev.c

@@ -505,7 +505,7 @@ static void bdi_remove_from_list(struct backing_dev_info *bdi)
 	list_del_rcu(&bdi->bdi_list);
 	spin_unlock_bh(&bdi_lock);
 
-	synchronize_rcu();
+	synchronize_rcu_expedited();
 }
 
 int bdi_register(struct backing_dev_info *bdi, struct device *parent,