Optimise page_count compound page test and make it consistent with
similar functions.

Signed-off-by: Nick Piggin <npiggin@suse.de>

Index: linux-2.6/include/linux/mm.h
===================================================================
--- linux-2.6.orig/include/linux/mm.h
+++ linux-2.6/include/linux/mm.h
@@ -313,7 +313,7 @@ extern void FASTCALL(__page_cache_releas
 
 static inline int page_count(struct page *page)
 {
-	if (PageCompound(page))
+	if (unlikely(PageCompound(page)))
 		page = (struct page *)page_private(page);
 	return atomic_read(&page->_count);
 }