blktrace support: discard notify messages

They are not interesting to us (they contain things like timestamps
and process information).

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/blktrace.c b/blktrace.c
index 8ba538a..5825d29 100644
--- a/blktrace.c
+++ b/blktrace.c
@@ -133,11 +133,7 @@
 		return;
 	if (t->action & BLK_TC_ACT(BLK_TC_PC))
 		return;
-
-	/*
-	 * should not happen, need to look into that...
-	 */
-	if (!t->bytes)
+	if (t->action & BLK_TC_ACT(BLK_TC_NOTIFY))
 		return;
 
 	rw = (t->action & BLK_TC_ACT(BLK_TC_WRITE)) != 0;