[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