block: fix use-after-free bug in blk throttle code

blk_throtl_exit() frees the throttle data hanging off the queue
in blk_cleanup_queue(), but blk_put_queue() will indirectly
dereference this data when calling blk_sync_queue() which in
turns calls throtl_shutdown_timer_wq().

Fix this by moving the freeing of the throttle data to when
the queue is truly being released, and post the call to
blk_sync_queue().

Reported-by: Ingo Molnar <mingo@elte.hu>
Tested-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

block/blk-core.c

@@ -462,8 +462,6 @@ void blk_cleanup_queue(struct request_queue *q)
 	if (q->elevator)
 		elevator_exit(q->elevator);
 
-	blk_throtl_exit(q);
-
 	blk_put_queue(q);
 }
 EXPORT_SYMBOL(blk_cleanup_queue);

block/blk-sysfs.c

@@ -471,6 +471,8 @@ static void blk_release_queue(struct kobject *kobj)
 
 	blk_sync_queue(q);
 
+	blk_throtl_exit(q);
+
 	if (rl->rq_pool)
 		mempool_destroy(rl->rq_pool);