Refactor start_process to hold file system lock for less time

src/userprog/process.c

@@ -149,9 +149,9 @@ start_process (void *proc_start_data)
       sema_up (&data->loaded);
       thread_exit ();
     }
-  thread_current ()->exec_file = exec_file;
   file_deny_write (exec_file);
   lock_release (&filesys_lock);
+  thread_current ()->exec_file = exec_file;
 
   data->success = load (data->file_name, &if_.eip, &if_.esp);