bpf: Relax log_buf NULL conditions when log_level>0 is requested

Drop the log_size>0 and log_buf!=NULL condition when log_level>0. This
allows users to request log_true_size of a full log without providing
actual (even if small) log buffer. Verifier log handling code was mostly
ready to handle NULL log->ubuf, so only few small changes were necessary
to prevent NULL log->ubuf from causing problems.

Note, that if user provided NULL log_buf with log_level>0 we don't
consider this a log truncation, and thus won't return -ENOSPC.

We also enforce that either (log_buf==NULL && log_size==0) or
(log_buf!=NULL && log_size>0).

Suggested-by: Lorenz Bauer <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Reviewed-by: Lorenz Bauer <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

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