blk-cgroup: fix leaks and online flag on radix_tree_insert failure

When radix_tree_insert() fails in blkg_create(), the error path has two
issues:

1. blkg->online is set to true unconditionally, even when the blkg was
   never fully inserted.  Move the assignment inside the success block.

2. The error path calls blkg_put() without first calling
   percpu_ref_kill().  Because the refcount is still in percpu mode,
   percpu_ref_put() only does this_cpu_sub() without checking for zero,
   so blkg_release() is never triggered.  This permanently leaks the
   blkg memory, its percpu iostat, policy data, the parent blkg
   reference, and the cgroup css reference — the latter preventing the
   cgroup from ever being destroyed.

Fix by replacing blkg_put() with percpu_ref_kill(), matching the pattern
used in blkg_destroy().

Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Tao Cui <cuitao@kylinos.cn>
Link: https://patch.msgid.link/20260715132407.1469777-1-cui.tao@linux.dev
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Tao Cui

block/blk-cgroup.c

@@ -434,15 +434,15 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg, struct gendisk *disk,
 				blkg->pd[i]->online = true;
 			}
 		}
+		blkg->online = true;
 	}
-	blkg->online = true;
 	spin_unlock(&blkcg->lock);
 
 	if (!ret)
 		return blkg;
 
 	/* @blkg failed fully initialized, use the usual release path */
-	blkg_put(blkg);
+	percpu_ref_kill(&blkg->refcnt);
 	return ERR_PTR(ret);
 
 err_free_blkg: