[Java] Correct the library loading for zlib in RocksJava.

Summary:
Correct the library loading for zlib in RocksJava: zlib should
be loaded by loadLibrary("z") instead of loadLibrary("zlib").

Test Plan:
make rocksdbjava
cd java
make db_bench
./jdb_bench.sh --compression_type=zlib

Reviewers: sdong, ljin, ankgup87

Reviewed By: ankgup87

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19341

java/org/rocksdb/RocksDB.java

@@ -21,7 +21,7 @@ import org.rocksdb.util.Environment;
 public class RocksDB extends RocksObject {
   public static final int NOT_FOUND = -1;
   private static final String[] compressionLibs_ = {
-      "snappy", "zlib", "bzip2", "lz4", "lz4hc"};
+      "snappy", "z", "bzip2", "lz4", "lz4hc"};
 
   /**
    * Loads the necessary library files.

java/org/rocksdb/benchmark/DbBenchmark.java

@@ -463,7 +463,7 @@ public class DbBenchmark {
       if (compressionType_.equals("snappy")) {
         System.loadLibrary("snappy");
       } else if (compressionType_.equals("zlib")) {
-        System.loadLibrary("zlib");
+        System.loadLibrary("z");
       } else if (compressionType_.equals("bzip2")) {
         System.loadLibrary("bzip2");
       } else if (compressionType_.equals("lz4")) {