add task_pending

jsg@ wants this for drm, and i've had a version of it in diffs sine
2016, but obviously havent needed to use it just yet.

task_pending is modelled on timeout_pending, and tells you if the
task is on a list waiting to execute.

ok jsg@

sys/kern/kern_task.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: kern_task.c,v 1.22 2017/12/14 00:45:16 dlg Exp $ */
+/*	$OpenBSD: kern_task.c,v 1.23 2018/12/16 03:36:02 dlg Exp $ */
 
 /*
  * Copyright (c) 2013 David Gwynne <dlg@openbsd.org>
@@ -24,8 +24,6 @@
 #include <sys/task.h>
 #include <sys/proc.h>
 
-#define TASK_ONQUEUE	1
-
 struct taskq {
 	enum {
 		TQ_S_CREATED,

sys/net/ifq.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: ifq.c,v 1.24 2018/12/11 01:36:42 dlg Exp $ */
+/*	$OpenBSD: ifq.c,v 1.25 2018/12/16 03:36:02 dlg Exp $ */
 
 /*
  * Copyright (c) 2015 David Gwynne <dlg@openbsd.org>
@@ -71,8 +71,6 @@ void	ifq_start_task(void *);
 void	ifq_restart_task(void *);
 void	ifq_barrier_task(void *);
 
-#define TASK_ONQUEUE 0x1
-
 void
 ifq_serialize(struct ifqueue *ifq, struct task *t)
 {

sys/sys/task.h

@@ -1,4 +1,4 @@
-/*	$OpenBSD: task.h,v 1.12 2017/11/13 23:52:49 dlg Exp $ */
+/*	$OpenBSD: task.h,v 1.13 2018/12/16 03:36:02 dlg Exp $ */
 
 /*
  * Copyright (c) 2013 David Gwynne <dlg@openbsd.org>
@@ -30,6 +30,8 @@ struct task {
 	unsigned int	t_flags;
 };
 
+#define TASK_ONQUEUE		1
+
 TAILQ_HEAD(task_list, task);
 
 #define TASKQ_MPSAFE		(1 << 0)
@@ -49,6 +51,8 @@ void		 task_set(struct task *, void (*)(void *), void *);
 int		 task_add(struct taskq *, struct task *);
 int		 task_del(struct taskq *, struct task *);
 
+#define task_pending(_t)	((_t)->t_flags & TASK_ONQUEUE)
+
 #endif /* _KERNEL */
 
 #endif /* _SYS_TASK_H_ */