ceph: pass parent inode info to ceph_encode_dentry_release if we have it

If we have a parent inode reference already, then we don't need to
go back up the directory tree to find one.

Link: http://tracker.ceph.com/issues/18148


Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Yan, Zheng <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>