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>

diff --cc drivers/md/md.c
Simple merge