s390/bpf: Fix 64-bit subtraction of the -0x80000000 constant

stable inclusion
from linux-4.19.207
commit e15c2fe2def24324bfdbfb7ec2837e40b2aac7fd
CVE: CVE-2021-20320

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

commit 6e61dc9d upstream.

The JIT uses agfi for subtracting constants, but -(-0x80000000) cannot
be represented as a 32-bit signed binary integer. Fix by using algfi in
this particular case.

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>