HDFS-12949. Fix findbugs warning in ImageWriter.java.

hadoop-tools/hadoop-fs2img/pom.xml

@@ -87,6 +87,16 @@
          </archive>
         </configuration>
        </plugin>
+      <plugin>
+        <groupId>org.codehaus.mojo</groupId>
+        <artifactId>findbugs-maven-plugin</artifactId>
+        <configuration>
+          <findbugsXmlOutput>true</findbugsXmlOutput>
+          <xmlOutput>true</xmlOutput>
+          <excludeFilterFile>${basedir}/dev-support/findbugs-exclude.xml</excludeFilterFile>
+          <effort>Max</effort>
+        </configuration>
+      </plugin>
     </plugins>
   </build>