async: make the final inode deletion an asynchronous event

this makes "rm -rf" on a (names cached) kernel tree go from
11.6 to 8.6 seconds on an ext3 filesystem

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>

include/linux/fs.h

@@ -1184,6 +1184,11 @@ struct super_block {
 	 * generic_show_options()
 	 */
 	char *s_options;
+
+	/*
+	 * storage for asynchronous operations
+	 */
+	struct list_head s_async_list;
 };
 
 extern struct timespec current_fs_time(struct super_block *sb);