[PATCH] md: Improve test for which raid1 device doesn't need to be written to.

Instead of testing last_used (which could change in unusual circumstances) we
test against the bdev that we read frmo, and don't write back to there.

drivers/md/raid1.c

@@ -825,7 +825,7 @@ static void sync_request_write(mddev_t *mddev, r1bio_t *r1_bio)
 		if (!conf->mirrors[i].rdev || 
 		    conf->mirrors[i].rdev->faulty)
 			continue;
-		if (i == conf->last_used)
+		if (conf->mirrors[i].rdev->bdev == bio->bi_bdev)
 			/*
 			 * we read from here, no need to write
 			 */