Add timestamp to netmap_slot

[kierank]

No description provided.

[giuseppelettieri]

We can't change the size of the slots (ABI break). Maybe we may reuse the ptr field when it is not already used for something else (e.g, offsets). Or, we may use offsets to reserve a place for the ts in the netmap buffer. What do you think?

[kierank]

Yes, on mlx5 we do exactly that, use ptr. But we want to start using ice which has ptr being used for offsets.
I'm happy to leave this here for the next ABI change you make because per packet timestamps are useful and now widely available from most modern NICs.