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 <dan.j.williams@intel.com>
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/md/md.c

@@ -1370,6 +1370,11 @@ static int bind_rdev_to_array(mdk_rdev_t * rdev, mddev_t * mddev)
 		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) {