s390/bpf: Fix optimizing out zero-extensions

stable inclusion
from linux-4.19.208
commit ddf58efd05b5d16d86ea4638675e8bd397320930
CVE: CVE-2021-20320

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

commit db7bee65 upstream.

Currently the JIT completely removes things like `reg32 += 0`,
however, the BPF_ALU semantics requires the target register to be
zero-extended in such cases.

Fix by optimizing out only the arithmetic operation, but not the
subsequent zero-extension.

Reported-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Fixes: 05462310 ("s390/bpf: Add s390x eBPF JIT compiler backend")
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: He Fengqing <hefengqing@huawei.com>
Reviewed-by: weiyang wang <wangweiyang2@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>