hpsa: fix 6-byte READ/WRITE with 0 length data xfer

a 6-byte READ/WRITE CDB with a 0 block data transfer really
means a 256 block data transfer.  The RAID mapping code failed
to handle this case.  For 10/12/16 byte READ/WRITEs, 0 just means
no data should be transferred, and should not trigger BUG_ON.
Signed-off-by: Stephen M. Cameron <scameron@beardog.cce.hp.com>
Reported-by: Robert Elliott <elliott@hp.com>
Reviewed-by: Robert Elliott <elliott@hp.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/scsi/hpsa.c

@@ -3686,6 +3686,8 @@ static int hpsa_scsi_ioaccel_raid_map(struct ctlr_info *h,
 			(((u64) cmd->cmnd[2]) << 8) |
 			cmd->cmnd[3];
 		block_cnt = cmd->cmnd[4];
+		if (block_cnt == 0)
+			block_cnt = 256;
 		break;
 	case WRITE_10:
 		is_write = 1;
@@ -3734,7 +3736,6 @@ static int hpsa_scsi_ioaccel_raid_map(struct ctlr_info *h,
 	default:
 		return IO_ACCEL_INELIGIBLE; /* process via normal I/O path */
 	}
-	BUG_ON(block_cnt == 0);
 	last_block = first_block + block_cnt - 1;
 
 	/* check for write to non-RAID-0 */