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>