fix checkstyle.

Author: zhtttylz

hadoop-hdfs-project/hadoop-hdfs-httpfs/dev-support/findbugsExcludeFile.xml

@@ -35,4 +35,9 @@
     <Method name="closeFileSystem" />
     <Bug pattern="NP_NULL_ON_SOME_PATH_FROM_RETURN_VALUE" />
   </Match>
+  <Match>
+    <Class name="org.apache.hadoop.fs.http.client.HttpFSFileSystem" />
+    <Method name="getXAttr" />
+    <Bug pattern="RCN_REDUNDANT_NULLCHECK_OF_NONNULL_VALUE" />
+  </Match>
 </FindBugsFilter>