samples/trace_printk: Wait for IRQ work to finish

trace_printk schedules work via irq_work_queue(), but doesn't
wait until it was processed. The kprobe_module.tc testcase does:

:;: "Load module again, which means the event1 should be recorded";:
modprobe trace-printk
grep "event1:" trace

so the grep which checks the trace file might run before the irq work
was processed. Fix this by adding a irq_work_sync().

Link: http://lore.kernel.org/linux-trace-devel/20191218074427.96184-3-svens@linux.ibm.com

Cc: stable@vger.kernel.org
Fixes: af2a0750f3 ("selftests/ftrace: Improve kprobe on module testcase to load/unload module")
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

samples/trace_printk/trace-printk.c

@@ -36,6 +36,7 @@ static int __init trace_printk_init(void)
 
 	/* Kick off printing in irq context */
 	irq_work_queue(&irqwork);
+	irq_work_sync(&irqwork);
 
 	trace_printk("This is a %s that will use trace_bprintk()\n",
 		     "static string");