mm/madvise: minor cleanup for swapin_walk_pmd_entry()

Passing index to pte_offset_map_lock() directly so the below calculation
can be avoided. Rename orig_pte to ptep as it's not changed. Also use
helper is_swap_pte() to improve the readability. No functional change
intended.

[akpm@linux-foundation.org: reduce scope of `ptep']
Link: https://lkml.kernel.org/r/20220618090527.37843-1-linmiaohe@huawei.comSigned-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/madvise.c

@@ -195,7 +195,6 @@ static int madvise_update_vma(struct vm_area_struct *vma,
 static int swapin_walk_pmd_entry(pmd_t *pmd, unsigned long start,
 	unsigned long end, struct mm_walk *walk)
 {
-	pte_t *orig_pte;
 	struct vm_area_struct *vma = walk->private;
 	unsigned long index;
 	struct swap_iocb *splug = NULL;
@@ -208,12 +207,13 @@ static int swapin_walk_pmd_entry(pmd_t *pmd, unsigned long start,
 		swp_entry_t entry;
 		struct page *page;
 		spinlock_t *ptl;
+		pte_t *ptep;
 
-		orig_pte = pte_offset_map_lock(vma->vm_mm, pmd, start, &ptl);
-		pte = *(orig_pte + ((index - start) / PAGE_SIZE));
-		pte_unmap_unlock(orig_pte, ptl);
+		ptep = pte_offset_map_lock(vma->vm_mm, pmd, index, &ptl);
+		pte = *ptep;
+		pte_unmap_unlock(ptep, ptl);
 
-		if (pte_present(pte) || pte_none(pte))
+		if (!is_swap_pte(pte))
 			continue;
 		entry = pte_to_swp_entry(pte);
 		if (unlikely(non_swap_entry(entry)))