Improve iflib busdma(9) KPI use.

Improve iflib busdma(9) KPI use.

- Specify BUS_DMA_NOWAIT for bus_dmamap_load() on rx refill, since
  callbacks are not supposed to be used.
- Match tso/non-tso tags to corresponding tx map operations.  Create
  separate tso maps for tx descriptors.  In particular, do not use
  non-tso tag to load, unload, or destroy a map created with tso tag.
- Add missed bus_dmamap_sync() calls.
  Submitted by: marius.

Reported and tested by:	pho
Reviewed by:	marius
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week