ATAng panic: ata_dmasetup: transfer active on this device!

I have a machine that panics reliably within ten minutes of
operation with ATAng.  It subsequently locks up, so I can't obtain
a dump, and I get a small amount of random filesystem corruption
upon rebooting.  Everything was fine with ATAog.

I don't have a serial cable handy at the moment, but the DDB
traceback looks like this:

	panic: ata_dmasetup: transfer active on this device!
	cpuid = 0
	[...]
	panic [...]
	ata_dmastart(d3075000,4000,0,101) [...]
	ata_pci_dmastart(d3075000,4000,0,20) [...]
	ata_transaction [...]
	ata_start [...]
	ata_completed [...]
	taskq_run [...]
	[...]

I can attach a serial cable and get some more information if
necessary.  The dual-processor machine in question has one 200GB
UATA 100 drive attached to ad0 and two 36GB SATA drives on ad4 and
ad6, mirrored through ccd(4).