pagewalk: fix walk_page_range() don't check find_vma() result properly

The doc of find_vma() says,

    /* Look up the first VMA which satisfies  addr < vm_end,  NULL if none. */
    struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr)
    {
     (snip)

Thus, caller should confirm whether the returned vma matches a desired one.
Signed-off-by: NKOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Hiroyuki Kamezawa <kamezawa.hiroyuki@gmail.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Matt Mackall <mpm@selenic.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/pagewalk.c

@@ -176,7 +176,7 @@ int walk_page_range(unsigned long addr, unsigned long end,
 		 * we can't handled it in the same manner as non-huge pages.
 		 */
 		vma = find_vma(walk->mm, addr);
-		if (vma && is_vm_hugetlb_page(vma)) {
+		if (vma && vma->vm_start <= addr && is_vm_hugetlb_page(vma)) {
 			if (vma->vm_end < next)
 				next = vma->vm_end;
 			/*