mm/hugetlb: fix use after free when subpool max_hpages accounting is not enabled

mm/hugetlb: fix use after free when subpool max_hpages accounting is not enabled

If a hugetlbfs filesystem is created with the min_size option and
without the size option, used_hpages is always 0 and might lead to
release subpool prematurely because it indicates no pages are used now
while there might be.

In order to fix this issue, we should check used_hpages == 0 iff
max_hpages accounting is enabled.  As max_hpages accounting should be
enabled in most common case, this is not worth a Cc stable.

[mike.kravetz@oracle.com: new changelog]

Link: https://lkml.kernel.org/r/20210126115510.53374-1-linmiaohe@huawei.com
Signed-off-by: Hongxiang Lou <louhongxiang@huawei.com>
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>