ftrace: avoid duplicated function when writing set_graph_function

Impact: fix a bug in function filter setting

when writing function to set_graph_function, we should check whether it
has existed in set_graph_function to avoid duplicating.

Signed-off-by: Liming Wang <liming.wang@windriver.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>