LU-14957 mdd: prepare xattrs before migration

LU-14957 mdd: prepare xattrs before migration

In directory migration, the xattrs should be prepared before starting
transaction, otherwise if remote MDT is down, which will cause local
MDT stuck as well.

Lustre-change: https://review.whamcloud.com/44741
Lustre-commit: dc1aa272d24cff9f06fd9ea71e4ad468c16acc52

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I79279e7b0c051a7542a71066fffd4ad70f559368
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/46188
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>