mm/hugetlb: fix WARN_ON(!kobj) in sysfs_create_group()

If sysfs_create_group() fails with hstate_attr_group, hstate_kobjs[hi]
will be set to NULL.  Then it will be passed to sysfs_create_group() if
h->demote_order != 0 thus triggering WARN_ON(!kobj) check.  Fix this by
making sure hstate_kobjs[hi] != NULL when calling sysfs_create_group.

Link: https://lkml.kernel.org/r/20220816130553.31406-3-linmiaohe@huawei.com
Fixes: 79dfc695 ("hugetlb: add demote hugetlb page sysfs interfaces")
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/hugetlb.c

@@ -3846,6 +3846,7 @@ static int hugetlb_sysfs_add_hstate(struct hstate *h, struct kobject *parent,
 	if (retval) {
 		kobject_put(hstate_kobjs[hi]);
 		hstate_kobjs[hi] = NULL;
+		return retval;
 	}
 
 	if (h->demote_order) {