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>