idletime: maintain cpuset over lifetime of idle thread

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/idletime.h b/idletime.h
index 819da25..bd6dcef 100644
--- a/idletime.h
+++ b/idletime.h
@@ -34,6 +34,8 @@
 	pthread_cond_t  cond;
 	pthread_mutex_t init_lock;
 	pthread_mutex_t start_lock;
+
+	os_cpu_mask_t cpu_mask;
 };
 
 struct idle_prof_common {