Fix slat numbers

After commit 306ddc9752eef70c3fbb111af63d197a3a6d447f, it was logging
the blocksize since arguments 3 and 4 were switched.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/io_u.c b/io_u.c
index 72f5ff2..126a9a8 100644
--- a/io_u.c
+++ b/io_u.c
@@ -1086,7 +1086,7 @@
 		unsigned long slat_time;
 
 		slat_time = utime_since(&io_u->start_time, &io_u->issue_time);
-		add_slat_sample(td, io_u->ddir, io_u->xfer_buflen, slat_time);
+		add_slat_sample(td, io_u->ddir, slat_time, io_u->xfer_buflen);
 	}
 }