BUG/MEDIUM: tasks: Decrement tasks_run_queue in tasklet_free().

If the tasklet is in the list, don't forget to decrement tasks_run_queue
in tasklet_free().

This should be backported to 1.9.
diff --git a/include/proto/task.h b/include/proto/task.h
index a1aa0ff..bd2fe4c 100644
--- a/include/proto/task.h
+++ b/include/proto/task.h
@@ -365,8 +365,10 @@
 
 static inline void tasklet_free(struct tasklet *tl)
 {
-	if (!LIST_ISEMPTY(&tl->list))
+	if (!LIST_ISEMPTY(&tl->list)) {
 		task_per_thread[tid].task_list_size--;
+		HA_ATOMIC_SUB(&tasks_run_queue, 1);
+	}
 	LIST_DEL(&tl->list);
 
 	pool_free(pool_head_tasklet, tl);