md: prevent duplicates in bind_rdev_to_array

Found when trying to reassemble an active externally managed array. Without
this check we hit the more noisy "sysfs duplicate" warning in the later call
to kobject_add.

Signed-off-by: Dan Williams <[e-mail]>
Signed-off-by: Neil Brown <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/md/md.c

···
 		MD_BUG();
 		return -EINVAL;
 	}
+
+	/* prevent duplicates */
+	if (find_rdev(mddev, rdev->bdev->bd_dev))
+		return -EEXIST;
+
 	/* make sure rdev->size exceeds mddev->size */
 	if (rdev->size && (mddev->size == 0 || rdev->size < mddev->size)) {
 		if (mddev->pers) {