[ARM] Fix use of page->count

Patch from: Ian Campbell

This changes the atomic_t in struct page named count into a private
member _count which breaks arch/arm/mm/init.c at line 80 which reads
page->count directly in show_mem().
                                                                                
The comments in the above changeset suggest that page_count(page) is
precisely equal to the old page->count semantics, even though the
semantics of _count are different, so I think the following is correct

arch/arm/mm/init.c

@@ -77,7 +77,7 @@ void show_mem(void)
 			else if (!page_count(page))
 				free++;
 			else
-				shared += atomic_read(&page->count) - 1;
+				shared += page_count(page) - 1;
 			page++;
 		} while (page < end);
 	}