HDFS-6460. Ignore stale and decommissioned nodes in NetworkTopology#sortByDistance. Contributed by Yongjun Zhang.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1602287 13f79535-47bb-0310-9956-ffa450edef68
HDFS-6395. Skip checking xattr limits for non-user-visible namespaces. Contributed by Yi Liu.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1602289 13f79535-47bb-0310-9956-ffa450edef68