mm/smaps: use vma->vm_pgoff directly when counting partial swap

As it's trying to cover the whole vma anyways, use direct vm_pgoff value
and vma_pages() rather than linear_page_index.

Link: https://lkml.kernel.org/r/20210917164756.8586-3-peterx@redhat.comSigned-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Hugh Dickins <hughd@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/shmem.c

@@ -856,9 +856,8 @@ unsigned long shmem_swap_usage(struct vm_area_struct *vma)
 		return swapped << PAGE_SHIFT;
 
 	/* Here comes the more involved part */
-	return shmem_partial_swap_usage(mapping,
-			linear_page_index(vma, vma->vm_start),
-			linear_page_index(vma, vma->vm_end));
+	return shmem_partial_swap_usage(mapping, vma->vm_pgoff,
+					vma->vm_pgoff + vma_pages(vma));
 }
 
 /*