Fix deadlock by release of lock and semaphore in the wrong order

src/userprog/process.c

@@ -262,8 +262,8 @@ process_exit (void)
          struct process_result *, and may be waiting so call sema_up */
       else
         {
-          sema_up (&cur->result->sema);
           lock_release (&cur->result->lock);
+          sema_up (&cur->result->sema);
         }
     }
 
@@ -283,8 +283,8 @@ process_exit (void)
       /* Child is still alive, signal via sema that parent has died. */
       else
         {
-          sema_up (&result->sema);
           lock_release (&result->lock);
+          sema_up (&result->sema);
         }
     }