HDFS-16687. RouterFsckServlet replicates code from DfsServlet base class (#4681)

[snmvaughan]

Backport from trunk. RouterFsckServlet replicates the method "getUGI(HttpServletRequest request, Configuration conf)" from DfsServlet instead of just extending DfsServlet.

• Does the title or this PR starts with the corresponding JIRA issue id (e.g. 'HADOOP-17799. Your PR title ...')?
• Object storage: have the integration tests been executed and the endpoint declared according to the connector-specific documentation?
• If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• If applicable, have you updated the LICENSE, LICENSE-binary, NOTICE-binary files?

[hadoop-yetus]

💔 -1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	10m 48s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 0s		No case conflicting files found.
+0 🆗	codespell	0m 0s		codespell was not available.
+0 🆗	detsecrets	0m 0s		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.
			_ branch-3.3 Compile Tests _
+1 💚	mvninstall	39m 16s		branch-3.3 passed
+1 💚	compile	0m 45s		branch-3.3 passed
+1 💚	checkstyle	0m 39s		branch-3.3 passed
+1 💚	mvnsite	0m 51s		branch-3.3 passed
+1 💚	javadoc	1m 13s		branch-3.3 passed
+1 💚	spotbugs	1m 39s		branch-3.3 passed
+1 💚	shadedclient	26m 43s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	0m 38s		the patch passed
+1 💚	compile	0m 33s		the patch passed
+1 💚	javac	0m 33s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	checkstyle	0m 21s		the patch passed
+1 💚	mvnsite	0m 37s		the patch passed
+1 💚	javadoc	0m 51s		the patch passed
+1 💚	spotbugs	1m 23s		the patch passed
+1 💚	shadedclient	26m 20s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	unit	15m 43s		hadoop-hdfs-rbf in the patch passed.
+1 💚	asflicense	0m 43s		The patch does not generate ASF License warnings.
		130m 32s

Subsystem	Report/Notes
Docker	ClientAPI=1.41 ServerAPI=1.41 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4790/1/artifact/out/Dockerfile
GITHUB PR	#4790
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient spotbugs checkstyle codespell detsecrets
uname	Linux 9680cefb252a 4.15.0-191-generic #202-Ubuntu SMP Thu Aug 4 01:49:29 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	branch-3.3 / 5284fe9
Default Java	Private Build-1.8.0_342-8u342-b07-0ubuntu1~18.04-b07
Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4790/1/testReport/
Max. process+thread count	1897 (vs. ulimit of 5500)
modules	C: hadoop-hdfs-project/hadoop-hdfs-rbf U: hadoop-hdfs-project/hadoop-hdfs-rbf
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4790/1/console
versions	git=2.17.1 maven=3.6.0 spotbugs=4.2.2
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