We can't depend on the test_bio being the same every time it is submitted.
Need to reset some of the fields in the bio each time.  [Kevin Corry]
--- diff/drivers/md/dm-mpath.c	2004-01-16 14:35:44.000000000 +0000
+++ source/drivers/md/dm-mpath.c	2004-01-16 14:36:29.000000000 +0000
@@ -215,6 +215,11 @@
 	if (down_trylock(&p->test_lock))
 		return;		/* last test io still pending */
 
+	p->test_bio->bi_sector = 0;
+	p->test_bio->bi_bdev = p->dev->bdev;
+	p->test_bio->bi_size = bdev_hardsect_size(p->dev->bdev);
+	p->test_bio->bi_idx = 0;
+
 	generic_make_request(p->test_bio);
 }