block: Fix a race between request queue removal and the block cgroup controller

block: Fix a race between request queue removal and the block cgroup controller

Avoid that the following race can occur:

blk_cleanup_queue()               blkcg_print_blkgs()
  spin_lock_irq(lock) (1)           spin_lock_irq(blkg->q->queue_lock) (2,5)
    q->queue_lock = &q->__queue_lock (3)
  spin_unlock_irq(lock) (4)
                                    spin_unlock_irq(blkg->q->queue_lock) (6)

(1) take driver lock;
(2) busy loop for driver lock;
(3) override driver lock with internal lock;
(4) unlock driver lock;
(5) can take driver lock now;
(6) but unlock internal lock.

This change is safe because only the SCSI core and the NVME core keep
a reference on a request queue after having called blk_cleanup_queue().
Neither driver accesses any of the removed data structures between its
blk_cleanup_queue() and blk_put_queue() calls.

Reported-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Jan Kara <jack@suse.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>