selftests: netfilter: fix current year

stable inclusion
from stable-5.10.17
commit ef8f281a653006e14b8d2999fe934e890e51e71b
bugzilla: 48169

--------------------------------

[ Upstream commit a3005b0f ]

use date %Y instead of %G to read current year
Problem appeared when running lkp-tests on 01/01/2021

Fixes: 48d072c4 ("selftests: netfilter: add time counter check")
Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>
Acked-by: Xie XiuQi <xiexiuqi@huawei.com>