Refactor ready_list_reinsert to require being called with interrupts disabled

src/threads/synch.c

@@ -220,7 +220,9 @@ donate_priority (struct thread *donee) {
           /* Only the sink node of the WFG isn't waiting for a lock and
              could be on the ready list. Thus, as its priority changed,
              it must be reinserted into the list. */
+          enum intr_level old_level = intr_disable ();
           ready_list_reinsert (donee);
+          intr_set_level (old_level);
           donee = NULL;
         }
       else