mds: reply -EOPNOTSUPP for unknown lock type

Signed-off-by: Yan, Zheng <zyan@redhat.com>
(cherry picked from commit 675392335c53ff7879031fb9184e4f35bcc90fe2)
Merge pull request #2740 from ceph/giant-unknown-locktype

mds: reply -EOPNOTSUPP for unknown lock type