divert: Use a better source identifier for netisr_queue_src() calls

[markj]

Summary

These opaque IDs are used by netisr to distribute work among threads.
The mapping function is simply SourceID % numthreads, so using socket
addresses as source IDs isn't going to distribute packets well due to
alignment.

Use the divert socket's generation number instead, as that suits this
purpose much better.

[zlei]

Good idea !