HDFS-17880 TreeWalk should add element to the last position

[houzhizhen]

Description of PR

The elements in FsDirectory are in alphabetical order. The new element should add to the last of queue.

How was this patch tested?

The existing testcases.

[hadoop-yetus]

💔 -1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	0m 39s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 0s		No case conflicting files found.
+0 🆗	codespell	0m 1s		codespell was not available.
+0 🆗	detsecrets	0m 1s		detect-secrets was not available.
+1 💚	@author	0m 0s		The patch does not contain any @author tags.
-1 ❌	test4tests	0m 0s		The patch doesn't appear to include any new or modified tests. Please justify why no new tests are needed for this patch. Also please list what manual steps were performed to verify this patch.
			_ trunk Compile Tests _
-1 ❌	mvninstall	27m 24s	/branch-mvninstall-root.txt	root in trunk failed.
+1 💚	compile	0m 31s		trunk passed with JDK Ubuntu-21.0.10+7-Ubuntu-124.04
+1 💚	compile	0m 28s		trunk passed with JDK Ubuntu-17.0.18+8-Ubuntu-124.04.1
+1 💚	checkstyle	0m 24s		trunk passed
+1 💚	mvnsite	0m 27s		trunk passed
+1 💚	javadoc	0m 29s		trunk passed with JDK Ubuntu-21.0.10+7-Ubuntu-124.04
+1 💚	javadoc	0m 27s		trunk passed with JDK Ubuntu-17.0.18+8-Ubuntu-124.04.1
+1 💚	spotbugs	0m 43s		trunk passed
-1 ❌	shadedclient	15m 11s		branch has errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	0m 13s		the patch passed
+1 💚	compile	0m 13s		the patch passed with JDK Ubuntu-21.0.10+7-Ubuntu-124.04
+1 💚	javac	0m 13s		the patch passed
+1 💚	compile	0m 12s		the patch passed with JDK Ubuntu-17.0.18+8-Ubuntu-124.04.1
+1 💚	javac	0m 12s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	checkstyle	0m 10s		the patch passed
+1 💚	mvnsite	0m 13s		the patch passed
+1 💚	javadoc	0m 12s		the patch passed with JDK Ubuntu-21.0.10+7-Ubuntu-124.04
+1 💚	javadoc	0m 13s		the patch passed with JDK Ubuntu-17.0.18+8-Ubuntu-124.04.1
+1 💚	spotbugs	0m 29s		the patch passed
-1 ❌	shadedclient	14m 34s		patch has errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	unit	0m 21s		hadoop-fs2img in the patch passed.
-1 ❌	asflicense	0m 24s	/results-asflicense.txt	The patch generated 1 ASF License warnings.
		66m 4s

Subsystem	Report/Notes
Docker	ClientAPI=1.53 ServerAPI=1.53 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8240/1/artifact/out/Dockerfile
GITHUB PR	#8240
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient spotbugs checkstyle codespell detsecrets
uname	Linux ca218d849fb8 5.15.0-164-generic #174-Ubuntu SMP Fri Nov 14 20:25:16 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / 31990a0
Default Java	Ubuntu-17.0.18+8-Ubuntu-124.04.1
Multi-JDK versions	/usr/lib/jvm/java-21-openjdk-amd64:Ubuntu-21.0.10+7-Ubuntu-124.04 /usr/lib/jvm/java-17-openjdk-amd64:Ubuntu-17.0.18+8-Ubuntu-124.04.1
Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8240/1/testReport/
Max. process+thread count	612 (vs. ulimit of 5500)
modules	C: hadoop-tools/hadoop-fs2img U: hadoop-tools/hadoop-fs2img
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8240/1/console
versions	git=2.43.0 maven=3.9.11 spotbugs=4.9.7
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