LU-4684 migrate: shrink dir layout after migration

LU-4684 migrate: shrink dir layout after migration

Use setxattr(XATTR_NAME_LMV) to shrink dir layout after migration,
this may change dir fid because it may shrink dir to 1-stripe, if so
replace this dir with its stripe.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I4fd3e83e188ac0c6c4845183c17701276fae94a8
Reviewed-on: https://review.whamcloud.com/31626
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>