Use the LZ4 algorithm to compress metadata when the corresponding feature is enabled

*** 20,30 ****
   */
  
  /*
   * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
   * Copyright (c) 2011, 2014 by Delphix. All rights reserved.
!  * Copyright 2013 Nexenta Systems, Inc.  All rights reserved.
   */
  
  /*
   * SPA: Storage Pool Allocator
   *
--- 20,30 ----
   */
  
  /*
   * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
   * Copyright (c) 2011, 2014 by Delphix. All rights reserved.
!  * Copyright (c) 2013, 2014, Nexenta Systems, Inc.  All rights reserved.
   */
  
  /*
   * SPA: Storage Pool Allocator
   *
*** 6104,6113 ****
--- 6104,6123 ----
  
          if (spa->spa_ubsync.ub_version < SPA_VERSION_FEATURES &&
              spa->spa_uberblock.ub_version >= SPA_VERSION_FEATURES) {
                  spa_feature_create_zap_objects(spa, tx);
          }
+ 
+         if (spa->spa_uberblock.ub_version >= SPA_VERSION_FEATURES) {
+                 boolean_t lz4_en = spa_feature_is_enabled(spa,
+                     SPA_FEATURE_LZ4_COMPRESS);
+                 boolean_t lz4_ac = spa_feature_is_active(spa,
+                     SPA_FEATURE_LZ4_COMPRESS);
+ 
+                 if (lz4_en && !lz4_ac)
+                         spa_feature_incr(spa, SPA_FEATURE_LZ4_COMPRESS, tx);
+         }
          rrw_exit(&dp->dp_config_rwlock, FTAG);
  }
  
  /*
   * Sync the specified transaction group.  New blocks may be dirtied as