Fix wrap bug in mtime_since()

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/time.c b/time.c
index ef05094..4af84bc 100644
--- a/time.c
+++ b/time.c
@@ -48,16 +48,13 @@
 		usec += 1000000;
 	}
 
+	if (sec < 0 || (sec == 0 && usec < 0))
+		return 0;
+
 	sec *= 1000UL;
 	usec /= 1000UL;
 	ret = sec + usec;
 
-	/*
-	 * time warp bug on some kernels?
-	 */
-	if (ret < 0)
-		ret = 0;
-
 	return ret;
 }