mds: allow early reply when request's witness list is empty

Early reply should be Ok when there were slave requests, but all
of them were for acquiring locks.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: avoid adding replicas of target dentry to rename witnesses

When the rename target dentry is NULL, we can use MDentryLink messages
instead of slave requests to update its replicas.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>