mm/memory_failure: fix the missing pte_unmap() call

The paired pte_unmap() call is missing before the
dev_pagemap_mapping_shift() returns. So fix it.

David says:
"I guess this code never runs on 32bit / highmem, that's why we didn't
notice so far".

[[e-mail]: cleanup]

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Qi Zheng <[e-mail]>
Reviewed-by: David Hildenbrand <[e-mail]>
Cc: Naoya Horiguchi <[e-mail]>
Cc: Muchun Song <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/memory-failure.c

···
 		struct vm_area_struct *vma)
 {
 	unsigned long address = vma_address(page, vma);
+	unsigned long ret = 0;
 	pgd_t *pgd;
 	p4d_t *p4d;
 	pud_t *pud;
···
 	if (pmd_devmap(*pmd))
 		return PMD_SHIFT;
 	pte = pte_offset_map(pmd, address);
-	if (!pte_present(*pte))
-		return 0;
-	if (pte_devmap(*pte))
-		return PAGE_SHIFT;
-	return 0;
+	if (pte_present(*pte) && pte_devmap(*pte))
+		ret = PAGE_SHIFT;
+	pte_unmap(pte);
+	return ret;
 }
 
 /*