workqueue: Set worker->desc to workqueue name by default

Work functions can use set_worker_desc() to improve the visibility of
what the worker task is doing.  Currently, the desc field is unset at
the beginning of each execution and there is a separate field to track
the field is set during the current execution.

Instead of leaving empty till desc is set, worker->desc can be used to
remember the last workqueue the worker worked on by default and users
that use set_worker_desc() can override it to something more
informative as necessary.

This simplifies desc handling and helps tracking the last workqueue
that the worker exected on to improve visibility.

Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue_internal.h

@@ -31,7 +31,6 @@ struct worker {
 	struct work_struct	*current_work;	/* L: work being processed */
 	work_func_t		current_func;	/* L: current_work's fn */
 	struct pool_workqueue	*current_pwq; /* L: current_work's pwq */
-	bool			desc_valid;	/* ->desc is valid */
 	struct list_head	scheduled;	/* L: scheduled works */
 
 	/* 64 bytes boundary on 64bit, 32 on 32bit */