bus_dmamap_sync: bug?

[Maxime Villard <max%m00nbsd.net@localhost>]

Seems to me there is a bug in x86's bus_dmamap_sync(). When the DMA buffer
is an UIO, and we bounce, the 'offset' argument is not taken into account
properly.

We should do
	copy uio+off into bouncebuf+off
but we do
	copy uio+0 into bouncebuf+off

A quick glance shows that other architectures have the same problem, like
ARM32's _bus_dma_uiomove().

Maxime