HDFS-5372. In FSNamesystem, hasReadLock() returns false if the current thread holds the write lock (Contributed by Vinay)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1543029 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/branch-2' into branch-2-gd