proc: fix oops on invalid /proc/<pid>/maps access

When m_start returns an error, the seq_file logic will still call m_stop
with that error entry, so we'd better make sure that we check it before
using it as a vma.

Introduced by commit ec6fd8a4355c ("report errors in /proc/*/*map*
sanely"), which replaced NULL with various ERR_PTR() cases.

(On ia64, you happen to get a unaligned fault instead of a page fault,
since the address used is generally some random error code like -EPERM)

Reported-by: Anca Emanuel <anca.emanuel@gmail.com>
Reported-by: Tony Luck <tony.luck@intel.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Américo Wang <xiyou.wangcong@gmail.com>
Cc: Stephen Wilson <wilsons@start.ca>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
static int clear_refs_pte_range(pmd_t *pmd, unsigned long addr,
				unsigned long end, struct mm_walk *walk)
{
	struct vm_area_struct *vma = walk->private;
	pte_t *pte, ptent;
	spinlock_t *ptl;
	struct page *page;

	split_huge_page_pmd(walk->mm, pmd);

	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
	for (; addr != end; pte++, addr += PAGE_SIZE) {
		ptent = *pte;
		if (!pte_present(ptent))
			continue;

		page = vm_normal_page(vma, addr, ptent);
		if (!page)
			continue;

		/* Clear accessed and referenced bits. */
		ptep_test_and_clear_young(vma, addr, pte);
		ClearPageReferenced(page);
	}
	pte_unmap_unlock(pte - 1, ptl);
	cond_resched();
	return 0;
}

static int clear_refs_pte_range(pmd_t *pmd, unsigned long addr,
				unsigned long end, struct mm_walk *walk)
{
	struct vm_area_struct *vma = walk->private;
	pte_t *pte, ptent;
	spinlock_t *ptl;
	struct page *page;

	split_huge_page_pmd(walk->mm, pmd);

	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
	for (; addr != end; pte++, addr += PAGE_SIZE) {
		ptent = *pte;
		if (!pte_present(ptent))
			continue;

		page = vm_normal_page(vma, addr, ptent);
		if (!page)
			continue;

		/* Clear accessed and referenced bits. */
		ptep_test_and_clear_young(vma, addr, pte);
		ClearPageReferenced(page);
	}
	pte_unmap_unlock(pte - 1, ptl);
	cond_resched();
	return 0;
}
