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

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.

Fixes: 528eb2cb87bc ("s390/bpf: Implement arch_prepare_bpf_trampoline()")
Reported-by: Hari Bathini <hbathini@linux.ibm.com>
Closes: https://lore.kernel.org/bpf/20260312080113.843408-1-hbathini@linux.ibm.com/
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Tested-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20260313174807.581826-1-iii@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>