microblaze: 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 microblaze 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: Michal Simek <monstr@monstr.eu>
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/microblaze/kernel/ftrace.c

@@ -22,8 +22,7 @@
 void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr)
 {
 	unsigned long old;
-	int faulted, err;
-	struct ftrace_graph_ent trace;
+	int faulted;
 	unsigned long return_hooker = (unsigned long)
 				&return_to_handler;
 
@@ -63,18 +62,8 @@ void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr)
 		return;
 	}
 
-	err = ftrace_push_return_trace(old, self_addr, &trace.depth, 0, NULL);
-	if (err == -EBUSY) {
+	if (function_graph_enter(old, self_addr, 0, NULL))
 		*parent = old;
-		return;
-	}
-
-	trace.func = self_addr;
-	/* Only trace if the calling function expects to */
-	if (!ftrace_graph_entry(&trace)) {
-		current->curr_ret_stack--;
-		*parent = old;
-	}
 }
 #endif /* CONFIG_FUNCTION_GRAPH_TRACER */