Implement support for rx buffer packing. Enable it by default for T5

cards.

This is a T4 and T5 chip feature which lets the chip deliver multiple
Ethernet frames in a single buffer.  This is more efficient within the
chip, in the driver, and reduces wastage of space in rx buffers.

- Always allocate rx buffers from the jumbop zone, no matter what the
  MTU is.  Do not use the normal cluster refcounting mechanism.
- Reserve space for an mbuf and a refcount in the cluster itself and let
  the chip DMA multiple frames in the rest.
- Use the embedded mbuf for the first frame and allocate mbufs on the
  fly for any additional frames delivered in the cluster.  Each of these
  mbufs has a reference on the underlying cluster.

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