block: mtip32xx: Fix usage of dma_map_sg()

The dma_map_sg() can fail and, in case of failure, returns 0. If it
fails, mtip_hw_submit_io() returns an error.

The dma_unmap_sg() requires the nents parameter to be the same as the
one passed to dma_map_sg(). This patch saves the nents in
command->scatter_ents.

Fixes: 88523a61558a ("block: Add driver for Micron RealSSD pcie flash cards")
Signed-off-by: Thomas Fourier <[e-mail]>
Reviewed-by: Andy Shevchenko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

drivers/block/mtip32xx/mtip32xx.c

···
  * @dir      Direction (read or write)
  *
  * return value
- *	None
+ *	0	The IO completed successfully.
+ *	-ENOMEM	The DMA mapping failed.
  */
-static void mtip_hw_submit_io(struct driver_data *dd, struct request *rq,
-			      struct mtip_cmd *command,
-			      struct blk_mq_hw_ctx *hctx)
+static int mtip_hw_submit_io(struct driver_data *dd, struct request *rq,
+			     struct mtip_cmd *command,
+			     struct blk_mq_hw_ctx *hctx)
 {
 	struct mtip_cmd_hdr *hdr =
 		dd->port->command_list + sizeof(struct mtip_cmd_hdr) * rq->tag;
···
 	unsigned int nents;
 
 	/* Map the scatter list for DMA access */
-	nents = blk_rq_map_sg(rq, command->sg);
-	nents = dma_map_sg(&dd->pdev->dev, command->sg, nents, dma_dir);
+	command->scatter_ents = blk_rq_map_sg(rq, command->sg);
+	nents = dma_map_sg(&dd->pdev->dev, command->sg,
+			   command->scatter_ents, dma_dir);
+	if (!nents)
+		return -ENOMEM;
+
 
 	prefetch(&port->flags);
-
-	command->scatter_ents = nents;
 
 	/*
 	 * The number of retries for this command before it is
···
 	if (unlikely(port->flags & MTIP_PF_PAUSE_IO)) {
 		set_bit(rq->tag, port->cmds_to_issue);
 		set_bit(MTIP_PF_ISSUE_CMDS_BIT, &port->flags);
-		return;
+		return 0;
 	}
 
 	/* Issue the command to the hardware */
 	mtip_issue_ncq_command(port, rq->tag);
+
+	return 0;
 }
 
 /*
···
 
 	blk_mq_start_request(rq);
 
-	mtip_hw_submit_io(dd, rq, cmd, hctx);
+	if (mtip_hw_submit_io(dd, rq, cmd, hctx))
+		return BLK_STS_IOERR;
+
 	return BLK_STS_OK;
 }