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: KOSAKI 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>