mm/cdm: Fix bug when the vma can be NULL pointer, in mark_vma_cdm().

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>

mm/mempolicy.c

@@ -178,7 +178,7 @@ static inline void clr_vm_cdm(struct vm_area_struct *vma)
 static void mark_vma_cdm(nodemask_t *nmask,
 		struct page *page, struct vm_area_struct *vma)
 {
-	if (!page)
+	if (!page || !vma)
 		return;
 
 	if (vma->vm_flags & VM_CDM)