mm/page_ext: rename offset to index

Here, 'offset' means entry index in page_ext array.  Following patch
will use 'offset' for field offset in each entry so rename current
'offset' to prevent confusion.

Link: http://lkml.kernel.org/r/1471315879-32294-5-git-send-email-iamjoonsoo.kim@lge.com
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_ext.c

@@ -102,7 +102,7 @@ void __meminit pgdat_page_ext_init(struct pglist_data *pgdat)
 struct page_ext *lookup_page_ext(struct page *page)
 {
 	unsigned long pfn = page_to_pfn(page);
-	unsigned long offset;
+	unsigned long index;
 	struct page_ext *base;
 
 	base = NODE_DATA(page_to_nid(page))->node_page_ext;
@@ -119,9 +119,9 @@ struct page_ext *lookup_page_ext(struct page *page)
 	if (unlikely(!base))
 		return NULL;
 #endif
-	offset = pfn - round_down(node_start_pfn(page_to_nid(page)),
+	index = pfn - round_down(node_start_pfn(page_to_nid(page)),
 					MAX_ORDER_NR_PAGES);
-	return base + offset;
+	return base + index;
 }
 
 static int __init alloc_node_page_ext(int nid)