powerpc/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 powerpc 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.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: linuxppc-dev@lists.ozlabs.org
Cc: stable@kernel.org
Fixes: 03274a3f ("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/powerpc/kernel/trace/ftrace.c

@@ -950,7 +950,6 @@ int ftrace_disable_ftrace_graph_caller(void)
  */
 unsigned long prepare_ftrace_return(unsigned long parent, unsigned long ip)
 {
-	struct ftrace_graph_ent trace;
 	unsigned long return_hooker;
 
 	if (unlikely(ftrace_graph_is_dead()))
@@ -961,17 +960,7 @@ unsigned long prepare_ftrace_return(unsigned long parent, unsigned long ip)
 
 	return_hooker = ppc_function_entry(return_to_handler);
 
-	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;
-
+	if (!function_graph_enter(parent, ip, 0, NULL))
 		parent = return_hooker;
 out:
 	return parent;