tcp: avoid disclosing uninitialized data in tcp_log code

[chs]

Summary

Zero various structures completely before filling in parts of them,
since we copy these entire structures out to userspace later.
Found with KMSAN.

Sponsored by: Netflix
Assisted-By: Claude Code - Opus 5 (1M context)

[tuexen]

sys/netinet/tcp_log_buf.c

1910

This is not needed. optlen can't be negative.

[tuexen]

sys/netinet/tcp_log_buf.c

1907

Right now, sizeof(log_buf->tlb_opts) is TCP_MAXOLEN and therefore the optlen can't be larger than `TCP_MAXOLEN. So this is not needed.