mm/filemap.c: remove redundant test from find_get_pages_contig

After we establish a reference on the page, we check the pointer
continues to be in the correct position in i_pages.  Checking
page->index afterwards is unnecessary; if it were to change, then the
pointer to it from the page cache would also move.  The check used to be
done before grabbing a reference on the page which was racy (see commit
9cbb4cb ("mm: find_get_pages_contig fixlet")), but nobody noticed
that moving the check after grabbing the reference was redundant.

Link: http://lkml.kernel.org/r/20190107200224.13260-1-willy@infradead.org
Signed-off-by: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/filemap.c

@@ -1837,16 +1837,6 @@ unsigned find_get_pages_contig(struct address_space *mapping, pgoff_t index,
 		if (unlikely(page != xas_reload(&xas)))
 			goto put_page;
 
-		/*
-		 * must check mapping and index after taking the ref.
-		 * otherwise we can get both false positives and false
-		 * negatives, which is just confusing to the caller.
-		 */
-		if (!page->mapping || page_to_pgoff(page) != xas.xa_index) {
-			put_page(page);
-			break;
-		}
-
 		pages[ret] = page;
 		if (++ret == nr_pages)
 			break;