scsi: core: fix the dma_max_mapping_size call

We should only call dma_max_mapping_size for devices that have a DMA mask
set, otherwise we can run into a NULL pointer dereference that will crash
the system.

Also we need to do right shift to get the sectors from the size in bytes,
not a left shift.

Fixes: bdd17bde ("scsi: core: take the DMA max mapping size into account")
Reported-by: Bart Van Assche <bvanassche@acm.org>
Reported-by: Ming Lei <tom.leiming@gmail.com>
Tested-by: Guilherme G. Piccoli <gpiccoli@canonical.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/scsi_lib.c

@@ -1784,8 +1784,10 @@ void __scsi_init_queue(struct Scsi_Host *shost, struct request_queue *q)
 		blk_queue_max_integrity_segments(q, shost->sg_prot_tablesize);
 	}
 
-	shost->max_sectors = min_t(unsigned int, shost->max_sectors,
-			dma_max_mapping_size(dev) << SECTOR_SHIFT);
+	if (dev->dma_mask) {
+		shost->max_sectors = min_t(unsigned int, shost->max_sectors,
+				dma_max_mapping_size(dev) >> SECTOR_SHIFT);
+	}
 	blk_queue_max_hw_sectors(q, shost->max_sectors);
 	if (shost->unchecked_isa_dma)
 		blk_queue_bounce_limit(q, BLK_BOUNCE_ISA);