selftests/bpf: Fix verify_pkcs7_sig on s390x

Use bpf_probe_read_kernel() instead of bpf_probe_read(), which is not
defined on all architectures.

While at it, improve the error handling: do not hide the verifier log,
and check the return values of bpf_probe_read_kernel() and
bpf_copy_from_user().

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230128000650.1516334-10-iii@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>