trace_workqueue: remove cpu_workqueue_stats->first_entry

cpu_workqueue_stats->first_entry is useless because we can retrieve the
header of a cpu workqueue using:
if (&cpu_workqueue_stats->list == workqueue_cpu_stat(cpu)->list.next)

[ Impact: cleanup ]
Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Tom Zanussi <tzanussi@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>

kernel/trace/trace_workqueue.c

@@ -16,8 +16,6 @@
 /* A cpu workqueue thread */
 struct cpu_workqueue_stats {
 	struct list_head            list;
-/* Useful to know if we print the cpu headers */
-	bool		            first_entry;
 	int		            cpu;
 	pid_t			    pid;
 /* Can be inserted from interrupt or user context, need to be atomic */
@@ -103,8 +101,6 @@ static void probe_workqueue_creation(struct task_struct *wq_thread, int cpu)
 	cws->pid = wq_thread->pid;
 
 	spin_lock_irqsave(&workqueue_cpu_stat(cpu)->lock, flags);
-	if (list_empty(&workqueue_cpu_stat(cpu)->list))
-		cws->first_entry = true;
 	list_add_tail(&cws->list, &workqueue_cpu_stat(cpu)->list);
 	spin_unlock_irqrestore(&workqueue_cpu_stat(cpu)->lock, flags);
 }