io_u_mark_depth: just set index, don't fall through

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/io_u.c b/io_u.c
index 53921e8..9aab97f 100644
--- a/io_u.c
+++ b/io_u.c
@@ -363,17 +363,22 @@
 
 	switch (td->cur_depth) {
 	default:
-		index++;
+		index = 6;
+		break;
 	case 32 ... 63:
-		index++;
+		index = 5;
+		break;
 	case 16 ... 31:
-		index++;
+		index = 4;
+		break;
 	case 8 ... 15:
-		index++;
+		index = 3;
+		break;
 	case 4 ... 7:
-		index++;
+		index = 2;
+		break;
 	case 2 ... 3:
-		index++;
+		index = 1;
 	case 1:
 		break;
 	}