add bus_dmamap_syncs around tx ring handling.

add bus_dmamap_syncs around tx ring handling.

this copies the dmamem wrapper from ixl (which has been copied
around lots of other places too) so it makes the bus_dma ops around
them more consistent with other drivers. before this vmx would use
the bus dma api to allocate physical memory, but then free the
dmamap so it wasnt available for use with bus_dmamap_sync().