HDFS-7531. Improve the concurrent access on FsVolumeList (Lei Xu via Colin P. McCabe)
(cherry picked from commit 3b173d95171d01ab55042b1162569d1cf14a8d43)

Conflicts:
	hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/FsDatasetImpl.java