cxgbe(4): significant rx rework.

- More flexible cluster size selection, including the ability to fall
  back to a safe cluster size (PAGE_SIZE from zone_jumbop by default) in
  case an allocation of a larger size fails.
- A single get_fl_payload() function that assembles the payload into an
  mbuf chain for any kind of freelist.  This replaces two variants: one
  for freelists with buffer packing enabled and another for those without.
- Buffer packing with any sized cluster.  It was limited to 4K clusters
  only before this change.
- Enable buffer packing for TOE rx queues as well.
- Statistics and tunables to go with all these changes.  The driver's
  man page will be updated separately.

MFC after:	5 weeks