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