Use the DEFINE_SHOW_ATTRIBUTE() macro and simplify the code.

Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200919012252.171437-1-miaoqinglang@huawei.com