[PATCH] Fix YA bug in __page_cache_release

__page_cache_release() needs to check PG_lru inside the lock, because
page reclaim may have taken the page off the LRU while this CPU waits
on the lock.

That's three bugs in a single twenty-line function.  So far.

mm/swap.c

@@ -76,21 +76,19 @@ void lru_add_drain(void)
  */
 void __page_cache_release(struct page *page)
 {
+	unsigned long flags;
 	BUG_ON(page_count(page) != 0);
-	if (PageLRU(page)) {
-		unsigned long flags;
 
-		spin_lock_irqsave(&_pagemap_lru_lock, flags);
-		if (TestClearPageLRU(page)) {
-			if (PageActive(page))
-				del_page_from_active_list(page);
-			else
-				del_page_from_inactive_list(page);
-		}
-		if (page_count(page) != 0)
-			page = NULL;
-		spin_unlock_irqrestore(&_pagemap_lru_lock, flags);
+	spin_lock_irqsave(&_pagemap_lru_lock, flags);
+	if (TestClearPageLRU(page)) {
+		if (PageActive(page))
+			del_page_from_active_list(page);
+		else
+			del_page_from_inactive_list(page);
 	}
+	if (page_count(page) != 0)
+		page = NULL;
+	spin_unlock_irqrestore(&_pagemap_lru_lock, flags);
 	if (page)
 		__free_pages_ok(page, 0);
 }