Fixed RocksJava test failure of shouldSetTestCappedPrefixExtractor

Summary:
Fixed RocksJava test failure of shouldSetTestCappedPrefixExtractor
by adding the missing native implementation of
useCappedPrefixExtractor.

Test Plan:
make jclean
make rocksdbjava -j32
make jtest

Reviewers: igor, anthony, IslamAbdelRahman, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D43551

java/rocksjni/options.cc

@@ -2147,6 +2147,17 @@ void Java_org_rocksdb_ColumnFamilyOptions_useFixedLengthPrefixExtractor(
           static_cast<int>(jprefix_length)));
 }
 
+/*
+ * Method:    useCappedPrefixExtractor
+ * Signature: (JI)V
+ */
+void Java_org_rocksdb_ColumnFamilyOptions_useCappedPrefixExtractor(
+    JNIEnv* env, jobject jobj, jlong jhandle, jint jprefix_length) {
+  reinterpret_cast<rocksdb::ColumnFamilyOptions*>(jhandle)->
+      prefix_extractor.reset(rocksdb::NewCappedPrefixTransform(
+          static_cast<int>(jprefix_length)));
+}
+
 /*
  * Method:    setTableFactory
  * Signature: (JJ)V