Don't call bus_dmamap_load(9) and bus_dmamap_sync(9) on command xfers
if (AT_READ|AT_WRITE) in ata_c->flags is set but ata_c->bcount is zero.
Someone actually tries to put such a command and it causes
DIAGNOSTIC panic in x86/bus_dma.c:_bus_dmamap_sync().
I think bus_dma(9) API itself may allow calls with mapsize==0
but there are many MD code that asserts offset>=mapsize or len==0.

The problem is reported and fix is confirmed by Takuro KUBOTA
with XEN DOM0 kernel (which has options DIAGNOSTIC).

(tsutsui)