save files under construction
TODO: for HDFS-5428, since we cannot break the compatibility of
fsimage, we store part of the under-construction files that are only
in snapshots in this \"under-construction-file\" section. As a
temporary solution, we use \"/.reserved/.inodes/<inodeid>\" as their
paths, so that when loading fsimage we do not put them into the lease
map. In the future, we can remove this hack when we can bump the
layout version.