- don't fault in unmap_single() when unmapping a coherent buffer.
- make swiotlb_alloc_coherent() more resilient for devices with
  weird DMA masks
- make swiotlb_dma_supported() return a useful value
Signed-off-by: David Mosberger-Tang <davidm@hpl.hp.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>