Supporting smaller TCP_INFO shapes optionally relaxing get_opt_sized response validation with strict flag

[millermeares]

Summary

Relax getsockopt size checking for TCP_INFO whose struct ABI can vary across kernel versions.

Motivation

Some kernel socket options return versioned, kernel-defined structs whose size has grown over time. Older kernels legitimately return fewer bytes and update optlen accordingly, causing strict size == sizeof(T) checks to fail even though the returned data is valid. The problem here is inaccessibility of TCP_INFO from pingora on certain kernel versions.

Change

• Add an strict flag to get_opt_sized
• Continue enforcing exact size for fixed-size options, allowing shorter returns only when explicitly opted in by passing strict as false.
• Opting TCP_INFO into this new behavior.
• Preserve safety by rejecting any size > sizeof(T)

Notes

When smaller sizes are allowed, zeroed fields may indicate that a field is absent rather than a true zero value. Note that this ambiguity can already exist today: a matching struct size does not guarantee that the kernel provides the same set of fields as the userspace TCP_INFO definition.