tracing: fix sparse warning: attribute function with __acquires/__releases

Fix this sparse warning:

  kernel/trace/trace.c:458:9: warning: context imbalance in 'register_tracer' - unexpected unlock
Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/trace/trace.c

@@ -459,6 +459,8 @@ update_max_tr_single(struct trace_array *tr, struct task_struct *tsk, int cpu)
  * Register a new plugin tracer.
  */
 int register_tracer(struct tracer *type)
+__releases(kernel_lock)
+__acquires(kernel_lock)
 {
 	struct tracer *t;
 	int len;