blk-mq-debugfs: warn about possible deadlock

blk-mq-debugfs: warn about possible deadlock

Creating new debugfs entries can trigger fs reclaim, hence we can't do
this with queue frozen, meanwhile, other locks that can be held while
queue is frozen should not be held as well.

Signed-off-by: Yu Kuai <yukuai@fnnas.com>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>