workqueue: add function in event of workqueue_activate_work

The trace event "workqueue_activate_work" only print work struct.
However, function is the region of interest in a full sequence of work.
Current workqueue_activate_work trace event output:

    workqueue_activate_work: work struct ffffff88b4a0f450

With this change, workqueue_activate_work will print the function name,
align with workqueue_queue_work/execute_start/execute_end event.

    workqueue_activate_work: work struct ffffff80413a78b8 function=vmstat_update
Signed-off-by: Kassey Li <quic_yingangl@quicinc.com>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Tejun Heo <tj@kernel.org>

include/trace/events/workqueue.h

@@ -64,13 +64,15 @@ TRACE_EVENT(workqueue_activate_work,
 
 	TP_STRUCT__entry(
 		__field( void *,	work	)
+		__field( void *,	function)
 	),
 
 	TP_fast_assign(
 		__entry->work		= work;
+		__entry->function	= work->func;
 	),
 
-	TP_printk("work struct %p", __entry->work)
+	TP_printk("work struct %p function=%ps ", __entry->work, __entry->function)
 );
 
 /**