LU-11025 dne: support directory restripe

LU-11025 dne: support directory restripe

This patch adds directory restripe support:
* 'lfs setdirstripe -m -1 -c <stripe_count>' on an existed directory
  will change this directory layout, if 'stripe_count' is larger than
  current count, new stripes are allocated after current stripes,
  otherwise merge stripes of this directory, NB, if stripe count is
  unchanged, but hash type changed, it's treated as merging, but
  rehashing actually.
* mdt_restripe() ia added to restripe directory.
* mdd_dir_declare_layout_split() is added to split directory, which
  handles both plain and striped directory split.
* lod_dir_declare_layout_split() will handle the internal of directory
  split.
* directory merge is simple compared to split, which just records
  target stripe count in LMV, and update it.

NB. this patch only restripe directory, but doesn't add the code to
migrate sub files, which will be implemented in the following patch.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I526f7423b909eb83cf8723e65981d713b3e42499
Reviewed-on: https://review.whamcloud.com/36898
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: Yingjin Qian <qian@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>