selftests/ftrace: Improve kprobe on module testcase to load/unload module

Improve kprobe events on module testcase to check module
load/unload with disabled/enabled events. This also change
the target module to trace_printk.ko, so it depends on
CONFIG_SAMPLE_TRACE_PRINTK=m.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>