function-graph: Allow writing the same val to set_graph_function

# echo 'do_open' > set_graph_function
 # echo 'do_open' >> set_graph_function
 bash: echo: write error: Invalid argument

Make it valid to write the same value to set_graph_function,
which is consistent with set_ftrace_filter interface.
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
LKML-reference: <4B1DC4E1.1060303@cn.fujitsu.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>

kernel/trace/ftrace.c

@@ -2552,11 +2552,10 @@ ftrace_set_func(unsigned long *array, int *idx, char *buffer)
 					exists = true;
 					break;
 				}
-			if (!exists) {
+			if (!exists)
 				array[(*idx)++] = rec->ip;
 			found = 1;
 		}
-		}
 	} while_for_each_ftrace_rec();
 
 	mutex_unlock(&ftrace_lock);