scsi: xen-scsifront: mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases where
we are expecting to fall through.

Notice that, in this particular case, I replaced "Missed the backend's
Closing state -- fallthrough" with "fall through - Missed the backend's
Closing state", which contains the "fall through" annotation at the
beginnig of the code comment, which is what GCC is expecting to find.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/xen-scsifront.c

@@ -1112,7 +1112,7 @@ static void scsifront_backend_changed(struct xenbus_device *dev,
 	case XenbusStateClosed:
 		if (dev->state == XenbusStateClosed)
 			break;
-		/* Missed the backend's Closing state -- fallthrough */
+		/* fall through - Missed the backend's Closing state */
 	case XenbusStateClosing:
 		scsifront_disconnect(info);
 		break;