mds: handle interaction between slave rollback and fragmenting

mds: handle interaction between slave rollback and fragmenting

For slave rename and rmdir events, the MDS needs to preserve non-auth
dirfrag where the renamed inode originally lives in until slave commit
event is encountered. Current method to handle this is use MDCache::
uncommitted_slave_rename_olddir to track any non-auth dirfrag that
need to be preserved. This method does not works well if any preserved
dirfrag gets fragmented by log event (such as ESubtreeMap) between the
slave prepare event and the slave commit event.

The fix is tracking inode of dirfrag instead of tracking dirfrag that
need to preserved directly.

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