riscv: stacktrace: fix usage of ftrace_graph_ret_addr()

ftrace_graph_ret_addr() takes an `idx` integer pointer that is used to
optimize the stack unwinding. Pass it a valid pointer to utilize the
optimizations that might be available in the future.

The commit is making riscv's usage of ftrace_graph_ret_addr() match
x86_64.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Link: https://lore.kernel.org/r/20240618145820.62112-1-puranjay@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/kernel/stacktrace.c

@@ -32,6 +32,7 @@ void notrace walk_stackframe(struct task_struct *task, struct pt_regs *regs,
 			     bool (*fn)(void *, unsigned long), void *arg)
 {
 	unsigned long fp, sp, pc;
+	int graph_idx = 0;
 	int level = 0;
 
 	if (regs) {
@@ -68,7 +69,7 @@ void notrace walk_stackframe(struct task_struct *task, struct pt_regs *regs,
 			pc = regs->ra;
 		} else {
 			fp = frame->fp;
-			pc = ftrace_graph_ret_addr(current, NULL, frame->ra,
+			pc = ftrace_graph_ret_addr(current, &graph_idx, frame->ra,
 						   &frame->ra);
 			if (pc == (unsigned long)ret_from_exception) {
 				if (unlikely(!__kernel_text_address(pc) || !fn(arg, pc)))