tracing/function-return-tracer: free the return stack on free_task()

Impact: avoid losing some traces when a task is freed

do_exit() is not the last function called when a task finishes.
There are still some functions which are to be called such as
ree_task().  So we delay the freeing of the return stack to the
last moment.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/kernel/fork.c b/kernel/fork.c
index fbf4a4c..d6e1a32 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -140,6 +140,7 @@
 	prop_local_destroy_single(&tsk->dirties);
 	free_thread_info(tsk->stack);
 	rt_mutex_debug_task_free(tsk);
+	ftrace_retfunc_exit_task(tsk);
 	free_task_struct(tsk);
 }
 EXPORT_SYMBOL(free_task);