selftests/ftrace: Do not trace do_softirq because of PREEMPT_RT

The PREEMPT_RT patchset does not use do_softirq() function thus trying
to filter for do_softirq fails for such kernel:

  echo do_softirq
  ftracetest: 81: echo: echo: I/O error

Choose some other visible function for the test.  The function does not
have to be actually executed during the test, because it is only testing
filter API interface.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>