md: unlock mddev_lock on an error path.

This error path retuns while still holding the lock - bad.

Fixes: 6791875e2e53 ("md: make reconfig_mutex optional for writes to md sysfs files.")
Cc: stable@vger.kernel.org (v4.0+)
Signed-off-by: NeilBrown <neilb@suse.com>

drivers/md/md.c

@@ -4014,8 +4014,10 @@ new_dev_store(struct mddev *mddev, const char *buf, size_t len)
 	else
 		rdev = md_import_device(dev, -1, -1);
 
-	if (IS_ERR(rdev))
+	if (IS_ERR(rdev)) {
+		mddev_unlock(mddev);
 		return PTR_ERR(rdev);
+	}
 	err = bind_rdev_to_array(rdev, mddev);
  out:
 	if (err)