gfs2: Update rl_unlinked before releasing rgrp lock

Function gfs2_free_di was changing the rgrp lvb count of unlinked
dinodes after the lock was released. This patch moves it inside the
lock.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>