selftests/bpf: Increase verifier log limit in veristat

The current default buffer size of 16MB allocated by veristat is no
longer sufficient to hold the verifier logs of some production BPF
programs. To address this issue, we need to increase the verifier log
limit.
Commit 7a9f5c65 ("bpf: increase verifier log limit") has already
increased the supported buffer size by the kernel, but veristat users
need to explicitly pass a log size argument to use the bigger log.

This patch adds a function to detect the maximum verifier log size
supported by the kernel and uses that by default in veristat.
This ensures that veristat can handle larger verifier logs without
requiring users to manually specify the log size.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20241023155314.126255-1-mykyta.yatsenko5@gmail.com