Fix botched merge with 355066

When merging from Netflix's tree, resetting the carsize was dropped
accidentally. This fix fixes that revision by properly resetting how
many are in the car.

Noticed by: mav@

sys/kern/subr_disk.c

@@ -201,6 +201,7 @@ bioq_insert_tail(struct bio_queue_head *head, struct bio *bp)
 
 	TAILQ_INSERT_TAIL(&head->queue, bp, bio_queue);
 	head->total++;
+	head->batched = 0;
 	head->insert_point = bp;
 	head->last_offset = bp->bio_offset;
 }