MINOR: tasks: Don't special-case when nbthreads == 1

Instead of checking if nbthreads == 1, just and thread_mask with
all_threads_mask to know if we're supposed to add the task to the local or
the global runqueue.
diff --git a/src/task.c b/src/task.c
index e357bc1..ce5b4f9 100644
--- a/src/task.c
+++ b/src/task.c
@@ -395,8 +395,7 @@
 			state = HA_ATOMIC_AND(&t->state, ~TASK_RUNNING);
 			if (state)
 #ifdef USE_THREAD
-				__task_wakeup(t, (t->thread_mask == tid_bit ||
-				    global.nbthread == 1) ?
+				__task_wakeup(t, ((t->thread_mask & all_threads_mask) == tid_bit) ?
 				    &rqueue_local[tid] : &rqueue);
 #else
 				__task_wakeup(t, &rqueue_local[tid]);