hulk inclusion
category: bugfix
bugzilla: 16069
CVE: NA
-------------------

alloc_page_vma() can be called by try_to_unuse(), in this case,
the vma is NULL.
Signed-off-by: NLijun Fang <fanglijun3@huawei.com>
Reviewed-by: NLi Zefan <lizefan@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>