RBF: getMountPointDates should fetch latest subdir time/date when parent dir is not present but /parent/child dirs are present in mount table

HDFS-13386 is not handling the case when /parent in not present in mount table but /parent/subdir is in mount table.

In this case getMountPointDates is not able to fetch the latest time for /parent as /parent is not present in mount table.

For this scenario we will display latest modified subdir date/time as /parent modified time.