lkdtm/stackleak: Make the test more verbose

Make the stack erasing test more verbose about the errors that it
can detect.

Signed-off-by: Alexander Popov <alex.popov@linux.com>
Cc: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20200102234907.585508-1-alex.popov@linux.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>