mds: don't take mds_lock when doing journal operations

Now journaler and objecter have internal locks to protect themself, there is
no need to take the mds_lock before using journaler and objecter.

Signed-off-by: Yan, Zheng <zyan@redhat.com>
Merge pull request #2391 from ceph/wip-mds-lock

Wip mds lock

Reviewed-by: John Spray <john.spray@redhat.com>