mds: preserve non-auth/unlinked objects until slave commit

mds: preserve non-auth/unlinked objects until slave commit

The MDS should not trim objects in non-auth subtree immediately after
replaying a slave rename. Because the slave rename may require rollback
later and these objects are needed for rollback.

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