selftests/ftrace: Fix kprobe string testcase to not probe notrace function

Fix kprobe string argument testcase to not probe notrace
function. Instead, it probes tracefs function which must
be available with ftrace.

Link: http://lkml.kernel.org/r/153294607107.32740.1664854684396589624.stgit@devbox



Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>