4045 zfs write throttle & i/o scheduler performance work
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Christopher Siden <christopher.siden@delphix.com>

*** 1026,1036 ****
          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),
              DMU_OS_IS_L2COMPRESSIBLE(os), &zp, dmu_objset_write_ready,
!             dmu_objset_write_done, os, ZIO_PRIORITY_ASYNC_WRITE,
              ZIO_FLAG_MUSTSUCCEED, &zb);
  
          /*
           * Sync special dnodes - the parent IO for the sync is the root block
           */
--- 1026,1036 ----
          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),
              DMU_OS_IS_L2COMPRESSIBLE(os), &zp, dmu_objset_write_ready,
!             NULL, dmu_objset_write_done, os, ZIO_PRIORITY_ASYNC_WRITE,
              ZIO_FLAG_MUSTSUCCEED, &zb);
  
          /*
           * Sync special dnodes - the parent IO for the sync is the root block
           */