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 <[e-mail]>
Reviewed-by: Shuah Khan <[e-mail]>
Acked-by: Sebastian Andrzej Siewior <[e-mail]>
Reviewed-by: Steven Rostedt (Google) <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/ftrace/test.d/ftrace/func_set_ftrace_file.tc

···
 
 FILTER=set_ftrace_filter
 FUNC1="schedule"
-FUNC2="do_softirq"
+FUNC2="scheduler_tick"
 
 ALL_FUNCS="#### all functions enabled ####"