tracing/ftrace: add missing unlock in register_stat_tracer()

We should unlock all_stat_sessions_mutex before returning failure.
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/trace/trace_stat.c

@@ -73,8 +73,10 @@ int register_stat_tracer(struct tracer_stat *trace)
 	/* Already registered? */
 	mutex_lock(&all_stat_sessions_mutex);
 	list_for_each_entry_safe(node, tmp, &all_stat_sessions, session_list) {
-		if (node->ts == trace)
+		if (node->ts == trace) {
+			mutex_unlock(&all_stat_sessions_mutex);
 			return -EINVAL;
+		}
 	}
 	mutex_unlock(&all_stat_sessions_mutex);