HDFS-17394. [FGL] Remove unused WriteHoldCount of FSNamesystemLock

[ZanderXu]

public int getWriteHoldCount() {
  return this.fsLock.getWriteHoldCount(FSNamesystemLockMode.GLOBAL);
}

@Deprecated // dirLock is obsolete, use namesystem.fsLock instead
public int getWriteHoldCount() {
  return namesystem.getWriteHoldCount();
}

// sanity check.
if (!hadDirReadLock || !hadFsnReadLock || hadDirWriteLock ||
    hadFsnWriteLock || dir.getReadHoldCount() != 1 ||
    fsn.getReadHoldCount() != 1) {
  // cannot relinquish
  return false;
} 

getWriteHoldCount in FSNamesystem.java and FSDirectory.java is unused.
dir.getReadHoldCount() is useless as it's same as fsn.getReadHoldCount().

[xinglin]

can we trigger a new build?

[xinglin]

change lgtm. pending yetus build

[hadoop-yetus]

💔 -1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	0m 0s		Docker mode activated.
-1 ❌	patch	0m 19s		#6571 does not apply to HDFS-17384. Rebase required? Wrong Branch? See https://cwiki.apache.org/confluence/display/HADOOP/How+To+Contribute for help.

Subsystem	Report/Notes
GITHUB PR	#6571
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-6571/2/console
versions	git=2.34.1
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[ferhui]

@ZanderXu Thanks for this patch. @xinglin Thanks for reviewing it. Merged.