riscv: ftrace: correct the condition logic in function graph tracer

The condition should be logical NOT to assign the hook address to parent
address. Because the return value 0 of function_graph_enter upon
success.

Fixes: e949b6d (riscv/function_graph: Simplify with function_graph_enter())
Signed-off-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: stable@vger.kernel.org
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>

Author: zongbox

arch/riscv/kernel/ftrace.c

@@ -142,7 +142,7 @@ void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr,
 	 */
 	old = *parent;
 
-	if (function_graph_enter(old, self_addr, frame_pointer, parent))
+	if (!function_graph_enter(old, self_addr, frame_pointer, parent))
 		*parent = return_hooker;
 }