scsi: Suppress gcc 7 fall-through warnings reported with W=1

The conclusion of a recent discussion about the new warnings
reported by gcc 7 is that the new warnings reported when building
with W=1 should be suppressed. However, gcc 7 still warns about
fall-through in switch statements when building with W=1. Suppress
these warnings by annotating the SCSI core properly.

See also Linus Torvalds, Lots of new warnings with gcc-7.1.1, 11
July 2017 (https://www.mail-archive.com/[e-mail]/msg115428.html).

References: commit bd664f6b3e37 ("disable new gcc-7.1.1 warnings for now")
Signed-off-by: Bart Van Assche <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Cc: Hannes Reinecke <[e-mail]>
Cc: Johannes Thumshirn <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/scsi/scsi_error.c

···
 			set_host_byte(scmd, DID_ALLOC_FAILURE);
 			return SUCCESS;
 		}
+		/* FALLTHROUGH */
 	case COPY_ABORTED:
 	case VOLUME_OVERFLOW:
 	case MISCOMPARE:
···
 			return ADD_TO_MLQUEUE;
 		else
 			set_host_byte(scmd, DID_TARGET_FAILURE);
+		/* FALLTHROUGH */
 
 	case ILLEGAL_REQUEST:
 		if (sshdr.asc == 0x20 || /* Invalid command operation code */
···
 	switch (status_byte(scmd->result)) {
 	case GOOD:
 		scsi_handle_queue_ramp_up(scmd->device);
+		/* FALLTHROUGH */
 	case COMMAND_TERMINATED:
 		return SUCCESS;
 	case CHECK_CONDITION:
···
 			set_host_byte(scmd, DID_TIME_OUT);
 			return SUCCESS;
 		}
+		/* FALLTHROUGH */
 	case DID_NO_CONNECT:
 	case DID_BAD_TARGET:
 		/*
···
 		 * the case of trying to send too many commands to a
 		 * tagged queueing device.
 		 */
+		/* FALLTHROUGH */
 	case BUSY:
 		/*
 		 * device can't talk to us at the moment.  Should only
···
 		if (scmd->cmnd[0] == REPORT_LUNS)
 			scmd->device->sdev_target->expecting_lun_change = 0;
 		scsi_handle_queue_ramp_up(scmd->device);
+		/* FALLTHROUGH */
 	case COMMAND_TERMINATED:
 		return SUCCESS;
 	case TASK_ABORTED:
···
 		rtn = scsi_try_host_reset(scmd);
 		if (rtn == SUCCESS)
 			break;
-	default:
 		/* FALLTHROUGH */
+	default:
 		rtn = FAILED;
 		break;
 	}

drivers/scsi/scsi_ioctl.c

···
 		case NOT_READY:	/* This happens if there is no disc in drive */
 			if (sdev->removable)
 				break;
+			/* FALLTHROUGH */
 		case UNIT_ATTENTION:
 			if (sdev->removable) {
 				sdev->changed = 1;
 				result = 0;	/* This is no longer considered an error */
 				break;
 			}
-		default:	/* Fall through for non-removable media */
+			/* FALLTHROUGH -- for non-removable media */
+		default:
 			sdev_printk(KERN_INFO, sdev,
 				    "ioctl_internal_command return code = %x\n",
 				    result);