[ruby/timeout] Synchronize all accesses to @done

* So it is trivially correct.
* Performance seems the same overall.

https://github.com/ruby/timeout/commit/5e0d8e1637

lib/timeout.rb

@@ -67,11 +67,13 @@ module Timeout
       @message = message
 
       @mutex = Mutex.new
-      @done = false
+      @done = false # protected by @mutex
     end
 
     def done?
-      @done
+      @mutex.synchronize do
+        @done
+      end
     end
 
     def expired?(now)