Refactor the VAP transmit path code into a utility function that both
the normal and the mesh transmit paths can use.

The API is a bit horrible because it both consumes the mbuf and frees
the node reference regardless of whether it succeeds or not.
It's a hold-over from how the code behaves; it'd be nice to have it
not free the node reference / mbuf if TX fails and let the caller
decide what to do.