riscv/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 riscv 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: Greentime Hu <[e-mail]>
Cc: Alan Kao <[e-mail]>
Cc: [e-mail]
Fixes: 03274a3ffb449 ("tracing/fgraph: Adjust fgraph depth before calling trace return callback")
Reviewed-by: Palmer Dabbelt <[e-mail]>
Reviewed-by: Masami Hiramatsu <[e-mail]>
Signed-off-by: Steven Rostedt (VMware) <[e-mail]>

arch/riscv/kernel/ftrace.c

···
 {
 	unsigned long return_hooker = (unsigned long)&return_to_handler;
 	unsigned long old;
-	struct ftrace_graph_ent trace;
 	int err;
 
 	if (unlikely(atomic_read(&current->tracing_graph_pause)))
···
 	 */
 	old = *parent;
 
-	trace.func = self_addr;
-	trace.depth = current->curr_ret_stack + 1;
-
-	if (!ftrace_graph_entry(&trace))
-		return;
-
-	err = ftrace_push_return_trace(old, self_addr, &trace.depth,
-				       frame_pointer, parent);
-	if (err == -EBUSY)
-		return;
-	*parent = return_hooker;
+	if (function_graph_enter(old, self_addr, frame_pointer, parent))
+		*parent = return_hooker;
 }
 
 #ifdef CONFIG_DYNAMIC_FTRACE