Ensure inodes in active filesystem are not deleted during snapshot delete

After HDFS-13101, it was observed in one of our customer deployments that delete snapshot ends up cleaning up inodes from active fs which can be referred from only one snapshot as the isLastReference() check for the parent dir introduced in HDFS-13101 may return true in certain cases. The aim of this Jira to add a check to ensure if the Inodes are being referred in the active fs , should not get deleted while deletion of snapshot happens.