s390/bpf: Zero-extend bpf prog return values and kfunc arguments

In the Linux kernel, the following vulnerability has been resolved:

s390/bpf: Zero-extend bpf prog return values and kfunc arguments

s390x ABI requires callers to zero-extend unsigned arguments and sign-extend signed arguments, and callees to zero-extend unsigned return values and sign-extend signed return values.

s390 BPF JIT currently implements only sign extension. Fix this omission and implement zero extension too.