[JNI] Fixed a column family related compile error.

Summary:
Fixed the following column family related compile error.

./java/rocksjni/write_batch.cc:211:66: error: cannot initialize a parameter of type 'rocksdb::ColumnFamilyMemTables *' with an lvalue of type 'rocksdb::MemTable *'
  rocksdb::Status s = rocksdb::WriteBatchInternal::InsertInto(b, mem, &options);
                                                                   ^~~

Test Plan:
make jni
make jtest

Reviewers: igor, haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17577

java/rocksjni/write_batch.cc

@@ -208,7 +208,9 @@ jbyteArray Java_org_rocksdb_WriteBatchTest_getContents(
   rocksdb::MemTable* mem = new rocksdb::MemTable(cmp, options);
   mem->Ref();
   std::string state;
-  rocksdb::Status s = rocksdb::WriteBatchInternal::InsertInto(b, mem, &options);
+  rocksdb::ColumnFamilyMemTablesDefault cf_mems_default(mem, &options);
+  rocksdb::Status s =
+      rocksdb::WriteBatchInternal::InsertInto(b, &cf_mems_default);
   int count = 0;
   rocksdb::Iterator* iter = mem->NewIterator();
   for (iter->SeekToFirst(); iter->Valid(); iter->Next()) {