[kenrel] Ensure page tables are zeroed before use

I forgot to zero out pages used for page tables, which didn't come back
to bite me until testing on physical hardware..

src/kernel/page_table.cpp

@@ -181,6 +181,7 @@ page_table::get_table_page()
 	s_page_cache = s_page_cache->next;
 	--s_cache_count;
 
+	kutil::memset(page, 0, memory::frame_size);
 	return reinterpret_cast<page_table*>(page);
 }