bpf: Avoid incorrect -EFAULT error in BPF_LOG_KERNEL mode

If verifier log is in BPF_LOG_KERNEL mode, no log->ubuf is expected and
it stays NULL throughout entire verification process. Don't erroneously
return -EFAULT in such case.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Lorenz Bauer <lmb@isovalent.com>
Link: https://lore.kernel.org/bpf/20230406234205.323208-10-andrii@kernel.org

kernel/bpf/verifier.c

@@ -18863,7 +18863,7 @@ skip_full_check:
 	bpf_vlog_finalize(log);
 	if (log->level && bpf_vlog_truncated(log))
 		ret = -ENOSPC;
-	if (log->level && !log->ubuf) {
+	if (log->level && log->level != BPF_LOG_KERNEL && !log->ubuf) {
 		ret = -EFAULT;
 		goto err_release_maps;
 	}