selftests/ftrace: Fix to check the existence of set_ftrace_filter

If we run ftracetest on the kernel with CONFIG_DYNAMIC_FTRACE=n,
there is no set_ftrace_filter and all test cases are failed, because
reset_ftrace_filter() returns an error.
Let's check whether set_ftrace_filter exists in reset_ftrace_filter()
and clean up only set_ftrace_notrace in initialize_ftrace().

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