selftests/ftrace: Do not use arch dependent do_IRQ as a target function

Instead using arch-dependent do_IRQ, use do_softirq as a
target function.

Applying do_IRQ to set_ftrace_filter always fail on arm/arm64 and any
other architectures which don't define do_IRQ. So, instead of using
that, use do_softirq which is defined in kernel/softirq.c.
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

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

@@ -33,7 +33,7 @@ do_reset
 
 FILTER=set_ftrace_filter
 FUNC1="schedule"
-FUNC2="do_IRQ"
+FUNC2="do_softirq"
 
 ALL_FUNCS="#### all functions enabled ####"