Limit URO coalesced size

[mtfriesen]

We should limit the maximum URO payload size to, at most, the maximum USO payload size of the same NIC. This dramatically simplifies forwarding scenarios, and should not be an onerous requirement.

We could expand on this by allowing the OS to dynamically set the NIC's max URO coalescing size, but that imposes much more of a burden and the incremental gains are questionable.