blk-mq: don't create hctx debugfs dir until q->debugfs_dir is created

blk_mq_debugfs_register_hctx() can be called by blk_mq_update_nr_hw_queues
when gendisk isn't added yet, such as nvme tcp.

Fixes the warning of 'debugfs: Directory 'hctx0' with parent '/' already present!'
which can be observed reliably when running blktests nvme/005.

Fixes: 6cfc0081 ("blk-mq: no need to check return value of debugfs_create functions")
Reported-by: Yi Zhang <yi.zhang@redhat.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Tested-by: Yi Zhang <yi.zhang@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220711090808.259682-1-ming.lei@redhat.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq-debugfs.c

@@ -728,6 +728,9 @@ void blk_mq_debugfs_register_hctx(struct request_queue *q,
 	char name[20];
 	int i;
 
+	if (!q->debugfs_dir)
+		return;
+
 	snprintf(name, sizeof(name), "hctx%u", hctx->queue_num);
 	hctx->debugfs_dir = debugfs_create_dir(name, q->debugfs_dir);