bus_dmamap_sync the rx ring once per em_rxeof call, rather than for every
rx descriptor.

slightly tweak by matthew
tested on alpha and amd64
the way vnet works means mclgeti cant do its job. remove hte ifp
argument to MCLGETI to make that clear.
introduce the if_rxr api. it is intended to pull the rx ring accounting
out of the mbuf layer, and break the assumption that an interface will
only have a single ring per mbuf cluster size.

mpi@ is ok with moving this forward