The SecondaryNameNode may change the inode id of root while doing checkpoint

SNN will call FSNamesystem#clear while merging the fsimage to its local FSNamesystem, and the FSDirectory will be reset in this process. However, in FSDirectory#reset, a new inode id will be allocated and assigned to the new root.

To fix the problem, we can either pass in an inode id in FSDirectory#createRoot, or update the root id in FSImageFormat#updateRootAttr.