Realistically, the usnic BTL doesn't need to know anything about the
underlying transport except for its header length (so that it knows
where the payload begins in a received buffer). So remove the use of
the specific transport prefix union and just rely on the usnic verbs
extension to tell us what the header length is if we're using the
usNIC/UDP transport, or sizeof(struct ibv_grh) if we're using usNIC/L2
transport.
This commit was SVN r30914.
6710c2ef3f