3756 want lz4 support for metadata compression

*** 19,28 ****
--- 19,29 ----
   * CDDL HEADER END
   */
  /*
   * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
   * Copyright (c) 2013 by Delphix. All rights reserved.
+  * Copyright (c) 2013 Martin Matuska. All rights reserved.
   */
  
  /* Portions Copyright 2010 Robert Milkowski */
  
  #include <sys/cred.h>
*** 986,996 ****
          SET_BOOKMARK(&zb, os->os_dsl_dataset ?
              os->os_dsl_dataset->ds_object : DMU_META_OBJSET,
              ZB_ROOT_OBJECT, ZB_ROOT_LEVEL, ZB_ROOT_BLKID);
          arc_release(os->os_phys_buf, &os->os_phys_buf);
  
!         dmu_write_policy(os, NULL, 0, 0, &zp);
  
          zio = arc_write(pio, os->os_spa, tx->tx_txg,
              os->os_rootbp, os->os_phys_buf, DMU_OS_IS_L2CACHEABLE(os), &zp,
              dmu_objset_write_ready, dmu_objset_write_done, os,
              ZIO_PRIORITY_ASYNC_WRITE, ZIO_FLAG_MUSTSUCCEED, &zb);
--- 987,997 ----
          SET_BOOKMARK(&zb, os->os_dsl_dataset ?
              os->os_dsl_dataset->ds_object : DMU_META_OBJSET,
              ZB_ROOT_OBJECT, ZB_ROOT_LEVEL, ZB_ROOT_BLKID);
          arc_release(os->os_phys_buf, &os->os_phys_buf);
  
!         dmu_write_policy(os, NULL, 0, 0, &zp, tx->tx_txg);
  
          zio = arc_write(pio, os->os_spa, tx->tx_txg,
              os->os_rootbp, os->os_phys_buf, DMU_OS_IS_L2CACHEABLE(os), &zp,
              dmu_objset_write_ready, dmu_objset_write_done, os,
              ZIO_PRIORITY_ASYNC_WRITE, ZIO_FLAG_MUSTSUCCEED, &zb);