support building a single packet out of multiple rx descriptors.

support building a single packet out of multiple rx descriptors.

rge is a bit like intel nics where you can put small mbufs on the
rx ring and it will chain them together to make a jumbo packet.
except unlike intel you can use any sized mbufs for the rx descriptors.

this adds the handling of these packets to the rx path. we're still
putting 9k frames on the ring though.