s390/function_graph: Simplify with function_graph_enter()

The function_graph_enter() function does the work of calling the function
graph hook function and the management of the shadow stack, simplifying the
work done in the architecture dependent prepare_ftrace_return().

Have s390 use the new code, and remove the shadow stack management as well as
having to set up the trace structure.

This is needed to prepare for a fix of a design bug on how the curr_ret_stack
is used.

Acked-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Julian Wiedmann <jwi@linux.ibm.com>
Cc: linux-s390@vger.kernel.org
Cc: stable@kernel.org
Fixes: 03274a3ffb449 ("tracing/fgraph: Adjust fgraph depth before calling trace return callback")
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

arch/s390/kernel/ftrace.c

@@ -203,22 +203,13 @@ device_initcall(ftrace_plt_init);
  */
 unsigned long prepare_ftrace_return(unsigned long parent, unsigned long ip)
 {
-	struct ftrace_graph_ent trace;
-
 	if (unlikely(ftrace_graph_is_dead()))
 		goto out;
 	if (unlikely(atomic_read(&current->tracing_graph_pause)))
 		goto out;
 	ip -= MCOUNT_INSN_SIZE;
-	trace.func = ip;
-	trace.depth = current->curr_ret_stack + 1;
-	/* Only trace if the calling function expects to. */
-	if (!ftrace_graph_entry(&trace))
-		goto out;
-	if (ftrace_push_return_trace(parent, ip, &trace.depth, 0,
-				     NULL) == -EBUSY)
-		goto out;
-	parent = (unsigned long) return_to_handler;
+	if (!function_graph_enter(parent, ip, 0, NULL))
+		parent = (unsigned long) return_to_handler;
 out:
 	return parent;
 }