blkcg: avoid free blkcg_root when failed to alloc blkcg policy

this patch fix two errors, firstly avoid kfree blk_root, secondly not
free(blkcg) ,if blkcg alloc fail(blkcg == NULL), just unlock that mutex;
Signed-off-by: Nweiping zhang <zhangweiping@didichuxing.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

block/blk-cgroup.c

@@ -1067,7 +1067,7 @@ blkcg_css_alloc(struct cgroup_subsys_state *parent_css)
 		blkcg = kzalloc(sizeof(*blkcg), GFP_KERNEL);
 		if (!blkcg) {
 			ret = ERR_PTR(-ENOMEM);
-			goto free_blkcg;
+			goto unlock;
 		}
 	}
 
@@ -1111,8 +1111,10 @@ blkcg_css_alloc(struct cgroup_subsys_state *parent_css)
 	for (i--; i >= 0; i--)
 		if (blkcg->cpd[i])
 			blkcg_policy[i]->cpd_free_fn(blkcg->cpd[i]);
-free_blkcg:
-	kfree(blkcg);
+
+	if (blkcg != &blkcg_root)
+		kfree(blkcg);
+unlock:
 	mutex_unlock(&blkcg_pol_mutex);
 	return ret;
 }