LU-11025 dne: change dir layout via dt_layout_change

LU-11025 dne: change dir layout via dt_layout_change

This patch includes following changes:

* Use existing mo_layout_change()/dt_layout_change() interface to
  change directory layout, which is cleaner and easier to add new
  features. The supported layout change operations are:
  1. MD_LAYOUT_DETACH: detach stripes from directory master object.
  2. MD_LAYOUT_ATTACH: attach stripes after target's.
  3. MD_LAYOUT_SHRINK: destroy stripes from specific offset.

* Previously layout change is done in MDD layer if striped directory
  is remote, however LOD can handle remote striped directory the same
  as local one, NB, remember to initialize index operations before
  altering directory layout. In this way layer violation can be
  avoided.

* Use setxattr(XATTR_NAME_LMV, LU_XATTR_CREATE) to create stripes
  for directories, and '0' or 'LU_XATTR_REPLACE' to set or replace
  directory LMV instead of setxattr(XATTR_NAME_LMV".set").

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Iac226f9e0e504f19837e14e93cee2542a2cb5e3d
Reviewed-on: https://review.whamcloud.com/37634
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>