mm/rmap: use rmap_walk() in try_to_unmap()

Now, we have an infrastructure in rmap_walk() to handle difference from
variants of rmap traversing functions.

So, just use it in try_to_unmap().

In this patch, I change following things.

1. enable rmap_walk() if !CONFIG_MIGRATION.
2. mechanical change to use rmap_walk() in try_to_unmap().

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Hillf Danton <dhillf@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/ksm.c

@@ -1982,7 +1982,7 @@ again:
 				continue;
 
 			ret = try_to_unmap_one(page, vma,
-					rmap_item->address, flags);
+					rmap_item->address, (void *)flags);
 			if (ret != SWAP_AGAIN || !page_mapped(page)) {
 				anon_vma_unlock_read(anon_vma);
 				goto out;
@@ -1996,7 +1996,6 @@ out:
 	return ret;
 }
 
-#ifdef CONFIG_MIGRATION
 int rmap_walk_ksm(struct page *page, struct rmap_walk_control *rwc)
 {
 	struct stable_node *stable_node;
@@ -2054,6 +2053,7 @@ out:
 	return ret;
 }
 
+#ifdef CONFIG_MIGRATION
 void ksm_migrate_page(struct page *newpage, struct page *oldpage)
 {
 	struct stable_node *stable_node;