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

*** 22,31 ****
--- 22,32 ----
   * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
   * Copyright (c) 2012, 2014 by Delphix. All rights reserved.
   */
  /* Copyright (c) 2013 by Saso Kiselkov. All rights reserved. */
  /* Copyright (c) 2013, Joyent, Inc. All rights reserved. */
+ /* Copyright (c) 2014, Nexenta Systems, Inc. All rights reserved. */
  
  #include <sys/dmu.h>
  #include <sys/dmu_impl.h>
  #include <sys/dmu_tx.h>
  #include <sys/dbuf.h>
*** 42,51 ****
--- 43,53 ----
  #include <sys/zfs_ioctl.h>
  #include <sys/zap.h>
  #include <sys/zio_checksum.h>
  #include <sys/zio_compress.h>
  #include <sys/sa.h>
+ #include <sys/zfeature.h>
  #ifdef _KERNEL
  #include <sys/vmsystm.h>
  #include <sys/zfs_znode.h>
  #endif
  
*** 1588,1600 ****
          if (ismd) {
                  /*
                   * XXX -- we should design a compression algorithm
                   * that specializes in arrays of bps.
                   */
!                 compress = zfs_mdcomp_disable ? ZIO_COMPRESS_EMPTY :
!                     ZIO_COMPRESS_LZJB;
  
                  /*
                   * Metadata always gets checksummed.  If the data
                   * checksum is multi-bit correctable, and it's not a
                   * ZBT-style checksum, then it's suitable for metadata
                   * as well.  Otherwise, the metadata checksum defaults
--- 1590,1610 ----
          if (ismd) {
                  /*
                   * XXX -- we should design a compression algorithm
                   * that specializes in arrays of bps.
                   */
!                 boolean_t lz4_ac = spa_feature_is_active(os->os_spa,
!                     SPA_FEATURE_LZ4_COMPRESS);
  
+                 if (zfs_mdcomp_disable) {
+                         compress = ZIO_COMPRESS_EMPTY;
+                 } else if (lz4_ac) {
+                         compress = ZIO_COMPRESS_LZ4;
+                 } else {
+                         compress = ZIO_COMPRESS_LZJB;
+                 }
+ 
                  /*
                   * Metadata always gets checksummed.  If the data
                   * checksum is multi-bit correctable, and it's not a
                   * ZBT-style checksum, then it's suitable for metadata
                   * as well.  Otherwise, the metadata checksum defaults