block layer: elv_iosched_show should get elv_list_lock

elv_iosched_show function iterates other elv_list,
hence elv_list_lock should be got.

Also the question is: in elv_iosched_show, elv_iosched_store
q->elevator->elevator_type construction is used without locking q->queue_lock.
Is it expected?..
Signed-off-by: Vasily Tarasov <vtaras@openvz.org>
Cc: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

block/elevator.c

@@ -892,7 +892,7 @@ ssize_t elv_iosched_show(request_queue_t *q, char *name)
 	struct list_head *entry;
 	int len = 0;
 
-	spin_lock_irq(q->queue_lock);
+	spin_lock_irq(&elv_list_lock);
 	list_for_each(entry, &elv_list) {
 		struct elevator_type *__e;
 
@@ -902,7 +902,7 @@ ssize_t elv_iosched_show(request_queue_t *q, char *name)
 		else
 			len += sprintf(name+len, "%s ", __e->elevator_name);
 	}
-	spin_unlock_irq(q->queue_lock);
+	spin_unlock_irq(&elv_list_lock);
 
 	len += sprintf(len+name, "\n");
 	return len;