blk-mq: skip CPU offline notify on unmapped hctx

If an hctx has no software ctx mapped, blk_mq_map_swqueue() never
allocates tags and leaves hctx->tags NULL. The CPU hotplug offline
notifier can still run for that hctx, return early since hctx cannot
hold any requests.

Signed-off-by: Cong Zhang <[e-mail]>
Fixes: bf0beec0607d ("blk-mq: drain I/O when all CPUs in a hctx are offline")
Reviewed-by: Ming Lei <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

block/blk-mq.c

···
 			struct blk_mq_hw_ctx, cpuhp_online);
 	int ret = 0;
 
-	if (blk_mq_hctx_has_online_cpu(hctx, cpu))
+	if (!hctx->nr_ctx || blk_mq_hctx_has_online_cpu(hctx, cpu))
 		return 0;
 
 	/*