workqueue: tracing the name of the workqueue instead of it's address

This patch tracing workqueue name instead of it's address, the
new format is as follows.

workqueue_queue_work: work struct=0000000084e3df56 function=
drm_fb_helper_dirty_work workqueue=events req_cpu=256 cpu=1

This tell us to know which workqueue our work is queued.

Signed-off-by: Zqiang <qiang.zhang@windriver.com>
Signed-off-by: Tejun Heo <tj@kernel.org>