HDFS-6988. Improve HDFS-6581 eviction configuration (Xiaoyu Yao via Colin P. McCabe)
(cherry picked from commit a52eb4bc5fb21574859f779001ea9d95bf5207fe)

Conflicts:
	hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/FsDatasetImpl.java
(cherry picked from commit 2bcda17ee7dd6af06fd880e13dde2517aa724c12)