Use the dma_set_mask_and_coherent helper to set the DMA mask.  Rely
on the relatively recent change that setting a larger than required
mask will never fail to avoid the need for the boilerplate 32-bit
fallback code.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>