ktls: Improve handling of the bind_threads tunable a bit

[markj]

Summary

• Only check for empty domains if we actually tried to configure domain affinity in the first place. Otherwise setting bind_threads=1 will always cause the sysctl value to be reported as zero. This is harmless since the threads end up being bound, but it's confusing.
• Try to improve the sysctl description a bit.

[jhb]

I saw this the other day and almost thought that we should have separate knobs for "bind threads to cores" and "bind sessions to NUMA domains" rather than have ktls_bind_threads serve as a kind of tri-state. However, I think this approach is fine.