core, dma-direct: add a flag 32-bit dma limits

Various PCI bridges (VIA PCI, Xilinx PCIe) limit DMA to only 32-bits
even if the device itself supports more.  Add a single bit flag to
struct device (to be moved into the dma extension once we get to it)
to flag such devices and reject larger DMA to them.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>