scsi: core: take the DMA max mapping size into account

We need to limit the device's max_sectors to what the DMA mapping
implementation can support.  If not, we risk running out of swiotlb
buffers easily.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>