Add a mmap destroy function to cleanup all mmap hash table entries upon thread exit

src/threads/thread.c

@@ -397,6 +397,8 @@ thread_exit (void)
   process_exit ();
 #endif
 
+  mmap_destroy ();
+
   /* Remove thread from all threads list, set our status to dying,
      and schedule another process.  That process will destroy us
      when it calls thread_schedule_tail(). */