bpf: Simplify logging-related error conditions handling

Move log->level == 0 check into bpf_vlog_truncated() instead of doing it
explicitly. Also remove unnecessary goto in kernel/bpf/verifier.c.

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/20230406234205.323208-11-andrii@kernel.org