blk-mq: check kobject state_in_sysfs before deleting in blk_mq_unregister_hctx

In __blk_mq_update_nr_hw_queues() the return value of
blk_mq_sysfs_register_hctxs() is not checked. If sysfs creation for hctx
fails, later changing the number of hw_queues or removing disk will
trigger the following warning:

kernfs: can not remove 'nr_tags', no directory
WARNING: CPU: 2 PID: 637 at fs/kernfs/dir.c:1707 kernfs_remove_by_name_ns+0x13f/0x160
Call Trace:
remove_files.isra.1+0x38/0xb0
sysfs_remove_group+0x4d/0x100
sysfs_remove_groups+0x31/0x60
__kobject_del+0x23/0xf0
kobject_del+0x17/0x40
blk_mq_unregister_hctx+0x5d/0x80
blk_mq_sysfs_unregister_hctxs+0x94/0xd0
blk_mq_update_nr_hw_queues+0x124/0x760
nullb_update_nr_hw_queues+0x71/0xf0 [null_blk]
nullb_device_submit_queues_store+0x92/0x120 [null_blk]

kobjct_del() was called unconditionally even if sysfs creation failed.
Fix it by checkig the kobject creation statusbefore deleting it.

Fixes: 477e19dedc9d ("blk-mq: adjust debugfs and sysfs register when updating nr_hw_queues")
Signed-off-by: Li Nan <[e-mail]>
Reviewed-by: Yu Kuai <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

block/blk-mq-sysfs.c

···
 		return;
 
 	hctx_for_each_ctx(hctx, ctx, i)
-		kobject_del(&ctx->kobj);
+		if (ctx->kobj.state_in_sysfs)
+			kobject_del(&ctx->kobj);
 
-	kobject_del(&hctx->kobj);
+	if (hctx->kobj.state_in_sysfs)
+		kobject_del(&hctx->kobj);
 }
 
 static int blk_mq_register_hctx(struct blk_mq_hw_ctx *hctx)