sparc/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 sparc 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: "David S. Miller" <davem@davemloft.net>
Cc: sparclinux@vger.kernel.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/sparc/kernel/ftrace.c

@@ -126,20 +126,11 @@ unsigned long prepare_ftrace_return(unsigned long parent,
 				    unsigned long frame_pointer)
 {
 	unsigned long return_hooker = (unsigned long) &return_to_handler;
-	struct ftrace_graph_ent trace;
 
 	if (unlikely(atomic_read(&current->tracing_graph_pause)))
 		return parent + 8UL;
 
-	trace.func = self_addr;
-	trace.depth = current->curr_ret_stack + 1;
-
-	/* Only trace if the calling function expects to */
-	if (!ftrace_graph_entry(&trace))
-		return parent + 8UL;
-
-	if (ftrace_push_return_trace(parent, self_addr, &trace.depth,
-				     frame_pointer, NULL) == -EBUSY)
+	if (function_graph_enter(parent, self_addr, frame_pointer, NULL))
 		return parent + 8UL;
 
 	return return_hooker;